Water-based ink, ink jet recording method, ink cartridge, recording unit, ink jet recording apparatus, and image forming method

ABSTRACT

A water-based ink of the present invention comprises water, a plurality of different water-soluble organic solvents, and a water-insoluble coloring material, wherein the above water-soluble organic solvents consist of a good solvent to the above water-insoluble coloring material and a poor solvent to the above water-insoluble coloring material, and the total amount (% by mass) of the good solvents in the ink is represented by A and the total amount (% by mass) of the poor solvent in the ink is represented by B, the ratio of A to B ranges from 10:5 to 10:30 both inclusive, and when the respective Ka values of the above water-soluble organic solvents are determined by the Bristow method and compared with each other, a water-soluble organic solvent having the largest Ka value is the poor solvent. The pigment ink has a sufficiently large area factor even with a small amount of ink and provides an image of high OD.

This application is a continuation of International Application No.PCT/JP03/16942, filed on Dec. 26, 2003, which claims the benefit ofJapanese Patent Application Nos. 2002-382045 filed on Dec. 27, 2002,2003-127476 filed on May 2, 2003, 2003-127599 filed on May 2, 2003 and2003-425520 filed on Dec. 22, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a water-based ink that comprises awater-insoluble coloring material. More specifically, the presentinvention relates to a water-based ink, which is preferably used for arecording method and a recording apparatus using an ink jet recordingsystem, and further in an ink jet image forming method.

2. Related Background Art

It is known that ink containing a water-insoluble coloring material suchas as a pigment (pigmented ink) can provide images excellent in fastnesssuch as water resistance or light resistance. In recent years, varioustechniques have been proposed to improve the optical density of an imageformed with such ink. For example, there has been proposed a method forfurther improving image density, using an ink containing aself-dispersing carbon black and a certain salt (refer to, e.g.,Japanese Unexamined Patent Publication No. 2000-198955). There has beenalso proposed a technique to form a high quality image, in which an inkcomposition for ink jet recording, containing a pigment, a polymerparticle, a water-soluble organic solvent and water, is applied to arecording Medium with an aqueous solution containing a polyvalent metalto cause react-on between the ink composition and the polyvalent metalin the aqueous solution (see e.g., Japanese Unexamined PatentPublication No. 2000-63719). In these techniques, a pigment presentdispersed in the ink is forced to agglomerate on the surface of therecording medium to prevent penetration of the pigment into therecording medium whereby an image of a higher density than withconventional pigmented ink can be obtained.

However, as a result of intensive studies, the present inventors foundout that, as the pigment particles are agglomerate on a recording mediumin the above techniques, the area on the surface of a recording mediumto be covered with the coloring material when a certain amount of theink droplet Is used (so-called area factor) tends to be insufficient.This means that the above techniques require more ink to obtain the sameimage density in comparison with the conventional pigmented ink in whicha pigment is dispersed by a polymer dispersant or the like. This pointneeds further improvement. Although there is a method of obtaining alarge area factor with a small ink droplet by increasing permeability ofthe ink to a recording medium, the ink of increased permeability diffusenot only on the surface of the recording medium but also into the insidethereof, resulting in insufficient image density.

The present inventors studied both advantages and disadvantages of theconventional inks and analyzed the characteristics of the images formedwith such inks. They have found that, when an ink contains a coloringmaterial in a larger amount, there arise such problems that coloringmaterial remains in a larger amount on the surface of the recordingmedium, visually uneven dots are formed, or the coloring material is noteffectively used but wasted in the recording medium. The presentinventors have found that images superior to the conventional images canbe obtained by solving at least one of these technical problemssummarized below. The present invention solves at least one of thefollowing problems.

(1) When a pigment which exists in the state of dispersion in an ink isforced to agglomerate on the surface of a recording medium, the area ofthe surface of the recording medium which the coloring material cancover with an ink droplet of a predetermined volume (so-called areafactor) might be insufficient. In this case, the amount of ink necessaryto obtain the same image density increases.

(2) When the ink permeability is increased, the ink spreads not only onthe surface of a recording medium but also permeates in a direction ofthe thickness of the recording medium. Accordingly, the coloringmaterial cannot distribute at a high density near the surface of therecording medium, and a high image density cannot be obtained.

Thus, an object of the present invention is to provide a water-basedink, which can provides sufficient area even with a small droplet ofpigmented ink (large area factor), can provide an image having high OD(image density), and has a long-term storage stability.

Another object of the present invention is to provide an ink jetrecording method of using the above ink to form a high-definition imagewith high OD with a small amount of the ink.

Another object of the present invention is to provide an ink cartridge,a recording unit, and an ink jet recording apparatus, which arepreferably used for the above recording method.

Further, another object of the present invention is to provide an imageforming method in which color mixing (bleeding) is effectively preventedon the border between a black ink region and a color ink region withoutcausing feathering, when a color image in which different colors areadjacent to one another is recorded on a plain paper.

The technical concept of the present invention can be summarized asfollows: a water-based ink that comprises water, a plurality ofdifferent water-soluble organic solvents, and a water-insoluble coloringmaterial, wherein the above water-soluble organic solvents are a goodsolvent and a poor solvent For the water-insoluble coloring material tobe used, and when Ka values of the above water-soluble organic solventsare determined by the Bristow method, the above poor solvent has thelargest Ka value and diffuses along the surface of a recording medium inthe form of substantially a perfect circle before the good solventdiffuses, assisting agglomeration of the above water-insoluble coloringmaterial in the diffusion process. By this structure, the water-basedink of the present invention has an advantage that it is not necessaryto contain a large amount of the coloring material that is diffused andwasted in a recording medium without contributing to image density as inthe prior art. In addition, an image can be formed in an ideal state,that is, much of the coloring material is not located on the surface ofa recording medium and at the same time the coloring material does notreach the back side of the recording medium thereby enabling printing onthe both sides, As a result, an image with a high density is uniformlyformed on the surface of the recording medium.

SUMMARY OF THE INVENTION

The above-described objects are achieved by the present inventionmentioned below. That is to say, a water-based ink according to oneaspect of the present invention comprises water, a plurality ofdifferent water-soluble organic solvents, and a water-insoluble coloringmaterial, wherein the above water-soluble organic solvents consist of agood solvent to the above water-insoluble coloring material and a poorsolvent to the above water-insoluble coloring material, and when thetotal amount (% by mass) of the-good solvents in the ink is denoted as Aand the total amount (% by mass) of the poor solvents in the ink isdenoted as B, the ratio of A to B ranges from 10:5 to 10:30 bothinclusive, and when Ka values of the above water-soluble organicsolvents are determined by the Bristow method and the obtained valuesare compared with each other, the water-soluble organic solvent havingthe largest Ka value is a poor solvent.

In addition, a water-based ink according to another aspect of thepresent invention comprises water, a plurality of differentwater-soluble organic solvents, and a water-insoluble coloring material,wherein the above water-soluble organic solvents consist of a goodsolvent to the above water-insoluble coloring material and a poorsolvent to the above water-insoluble coloring material, and when Kavalues of the above water-soluble organic solvents is are determined bythe Bristow method and the obtained values are compared with each other,the water-soluble organic solvent having the largest Ka value is a poorsolvent, this is further characterized in that the attaching state ofthis water-based ink to a plain paper is defined as follows: when theink is dropped to a plain paper by using a needle with a needle diameterof 28 G (inner diameter: 0.18 mm, and outer diameter: 0.36 mm)positioned at a height of 4 mm above the surface of the plain paper, andfixed on the surface, the measurement value of a diameter of an ink dotobtained immediately after the ink impacted the plain paper denoted bydT, the measurement value of the largest diameter of the spread of theink after the ink fixed on the plain paper denoted by dS, and themeasurement value of the largest diameter of the spread of thewater-insoluble coloring material in the ink after the ink fixed on theplain paper denoted by dC satisfy a relationship shown below (Formula1):dC<dI<dS (Formula 1)and the penetration depth of the water-insoluble coloring material tothe plain paper is less than 30 μm after the ink is printed by the inkjet recording and fixed on the plain paper.

Moreover, in another aspect of the present invention, there is providedan ink jet recording method, which is characterized by a step ofejecting the water-based ink having the above structure by the ink jetmethod.

Furthermore, in another aspect of the present invention, there areprovided: all ink cartridge for storing the water-based ink having theabove structure; a recording unit comprising an ink storing unit forstoring the water-based ink having the above structure and an ink jethead for ejecting the ink; and an ink jet recording apparatus comprisingan ink storing unit for storing the water-based ink having the abovestructure and an ink jet head for ejecting the ink.

Still further, in another preferred aspect of the present invention,there is provided an image forming method for ink jet recording on aplain paper using a black ink and at least one water based color ink,characterized in that the water-based ink having the above structure isused as a black ink, and when an image in which an image formed with theblack ink and an image formed with a color ink are adjacent to isformed, the image is formed by scanning for attaching the black ink to ablack image, and then scanning for attaching the color ink to a regionin which the black image has been formed.

Still further, in another aspect of the present invention, there isprovided a water-based ink, which comprises water, a plurality ofdifferent water-soluble organic solvents, and a water-insoluble coloringmaterial, wherein the above water-soluble organic solvents consist of agood solvent to the above water-insoluble coloring material and a poorsolvent to the above water-insoluble coloring material, and when largestKa values of the above water-soluble organic solvents are determined bythe Bristow method, a water-soluble organic solvent having the greatestKa value is the poor solvent, and the poor solvent permeates a recordingmedium before the good solvent does, so that it assists agglomeration ofthe above water-insoluble coloring material in the good solvent on thesurface of the recording medium.

(Effect of the Invention)

According to the present invention, there is provided a water-based inkthat is a pigmented ink, which has a sufficiently large area factor evenfrom a small amount of ink droplets and provides an image with high OD(image density). In addition, according to the-present invention, usingsuch an ink, there are provided: an ink jet recording method for forminga high definition image with high OD even from a small additive amountof ink; an ink cartridge preferably used for the above recording method;a recording unit; and an ink jet recording apparatus. Moreover,according to the present invention, there is provided an image formingmethod in which color mixing (bleeding) is effectively prevented on theborder between a black ink region and a color ink region without causingfeathering, when a colored image in which different colors are adjacentto each other on a plain paper.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of an ink jet recordingapparatus head;

FIG. 2 is a cross-sectional view of the ink jet recording apparatushead;

FIG. 3 is a perspective view of the appearance of a head constituted ofseveral heads as shown in FIG. 1;

FIG. 4 is a perspective view showing an example of an ink jet recordingapparatus;

FIG. 5 is a longitudinal sectional view of an ink cartridge;

FIG. 6 is a perspective view showing an example of a recording unit;

FIG. 7 is a view showing an example of the structure of a recordinghead;

FIG. 8 is an example of a recording head used in the present invention;

FIG. 9 is another example of a recording head used in the presentinvention;

FIG. 10 is another example of a recording head used in the presentinvention;

FIG. 11 is another example of a recording head used in the presentinvention;

FIG. 12 is another example of a recording head used in the presentinvention;

FIG. 13 is another example of a recording head used in the presentinvention;

FIGS. 14A, 14B, 14C and 14D schematically illustrate the states when anink droplet of the present invention landed on the surface of arecording medium;

FIG. 15 schematically illustrates the difference in the ink dots ofExamples and Comparative Examples after the ink fixed on a plain paper;and

FIG. 16 is a schematic view showing the difference in the distributionof a coloring material in a depth direction after the inks of certainExample and Comparative Example fixed on a plain paper.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described further in detail below,referring to preferred embodiments. First, a poor solvent and a goodsolvent used in the present specification are explained. The detaineddefinitions will be described later, but here, a water-soluble organicsolvent causing stable dispersion of the water-insoluble coloringmaterial therein is called a good solvent, and a water-soluble organicsolvent causing unstable dispersion of the water-insoluble coloringmaterial therein is called a poor solvent, regardless of the method ofdispersing a water-insoluble coloring material. The inventors of thepresent invention paid attention to water-soluble organic solventscontained in a water-based ink together with a water-insoluble coloringmaterial, and classified them into good solvents having a high abilityto dissolve or disperse the water-insoluble coloring material and poorsolvents having a poor ability of dissolving or dispersing the coloringmaterial. The present invention is characterized in that a water-basedink is designed such that the ratio between the poor solvent and thegood solvent in the ink is set within a certain range. By adopting sucha structure, an image with reduced feathering and bleeding can beobtained even on a plain paper having various problems in imageformation with conventional water-based ink. The present inventionprovides an ink having a sufficiently large area factor (forms a largearea with a small amount of ink droplets) and being capable of formingan image with high OD. Furthermore, the use of such ink enableshigh-speed printing, down-sizing of the recording apparatus, and costreduction including consumable supplies, and further, it realizes imagesof excellent fastness and higher print density. Finding out remarkableeffects such as formation of high quality images, the inventorscompleted the present invention.

The reason why such advantages can be obtained by the present inventionis not fully understood, but the present inventors speculate as follows.In general, when an image is formed with a water-based ink on arecording paper such as plain paper, it is necessary to make thecoloring material stay on the paper efficiently in order to realize highprint density and print quality. As a method of doing so, there is amethod of attaching a reaction solution to a recording medium and thenattaching a pigmented ink to the recording paper, so as to obtain anexcellent print density and print quality. There is also a method ofusing a special dispersant to achieve both the storage stability of inkand a high print density. However, according to studies of the presentinventors, it is still difficult to obtain a sufficient print density bythese methods. In particular, it is impossible to have a sufficientlylarge area factor with a small ink droplet as well as obtaining a highprint density.

The water-based ink of the present invention comprises at least water, awater-insoluble coloring material, and a plurality of differentwater-soluble organic solvents. The above water-soluble organic solventsinclude a good solvent to the above water-insoluble coloring materialand a poor solvent to the above water-insoluble coloring material. Whenthe water-based ink is in a liquid state where water, a water-insolublecoloring material and water-soluble organic solvents including both agood solvent and a poor solvent to the water-insoluble coloring materialare mixed at a certain ratio, the storage stability of thewater-insoluble coloring material such as a pigment is maintained.

When such a water-based ink of the present invention is printed on arecording median, and particularly on a plain paper, an extremelyexcellent print density and print quality can be obtained. The reason isspeculated as follows: as shown in FIG. 14A, when an ink droplet 1301 ofthe present invention is applied on a recording medium 1300 such as aslain paper, the component ratio of water, a water-insoluble coloringmaterial, and a good solvent and a poor solvent to the water-insolublecoloring material contained in the ink changes after the ink landed onthe recording medium. That is, as shown in FIGS. 14A and 14B, after theink droplet 1301 landed on the surface of the recording medium 1300, asthe ink is fixed to the recording medium, a poor solvent 1307 having ahigh Ka value diffuses first among the water-soluble organic solventswith evaporation of water, in the vicinity of the surface of therecording medium, in the form of substantially a perfect circle, andthen a good solvent with a low Ka value diffuses, so that an ink dot isformed.

FIGS. 14B to 14D schematically show the states of the ink after itlanded on the recording medium 1300 until it is fixed thereto. When thespread of an ink dot in this case is considered, the concentration ofthe poor solvent must be high at a periphery 1302 of a dot where the inkand the paper are in contact than at a center 1303 of the dot. As aresult, when the ink dot diffuses along the surface of the recordingmedium in the form of substantially a perfect circle, the concentrationof the poor solvent 1307 sharply increases, the water-insoluble coloringmaterial becomes unstable so that agglomeration of the coloring materialor destruction of the dispersion state occurs. As a result, forming anoutline that is substantially a perfect circle on the surface of thepaper (refer to FIG. 4B), the water-insoluble coloring material 1304stays near the surface of the recording medium 1300, and a bank of thewater-insoluble coloring material is formed on the periphery of the dot.In such a manner, it is considered that the dot of the water-insolublecoloring material is formed in the form of a perfect circle, and thatthe dot is immobilized on the paper in that state (refer to FIG. 14C).The dot formation with the water-insoluble coloring material iscompleted at this point, but the water-soluble organic solvents andwater contained In the ink still diffuse and spread in a radial form.That is to say, even aefter the dot formation with the water-insolublecoloring material, water and water-soluble organic solvents diffusealong the surface of the recording medium. Subsequently, thewater-soluble organic solvents evaporate from or penetrate into therecording medium at the center 1303 where the good solvent is rich, andthe water-insoluble coloring material also precipitates in this area, sothat a dot 1305 for image formation is made (refer to FIGS. 14A to 14D).An ink image formed by the above-described process has a sufficientlylarge area factor even from a small amount of ink droplets and has ahigh print density. Moreover, since generation of feathering issufficiently reduced, a high quality image can be obtained.

Under the above-assumed mechanism, the good solvent and poor solventused in the present invention are determined by the condition whether ornot it can favorably maintain the dispersed state of the water-insolublecoloring material. That is to say, the good solvent or poor solvent isdetermined depending on their relationship with the water-insolublecoloring material or a dispersant thereof. Accordingly, when a goodsolvent and a poor solvent are selected to prepare the ink of thepresent invention, it is preferable to observe the dispersed state orstability of a water-insoluble coloring material to be used with thesolvent, so as to select the good and poor solvents based on the resultsof the observation. The present inventors conducted various studiesregarding the criterion for determining good and poor solvents thatprovide the effects of the present invention based on the relationshipwith the effects of the present invention. A solution containingapproximately 50% by mass of a solvent to be determined and awater-insoluble coloring material to be used for the ink in a dispersedstate was held at 60° C. for 48 hours. A particle size in the abovesolution was compared with that of another pigment dispersion-solutioncontaining the same pigment but none or little of the solvent to bedetermined. If the particle size of the test solution becomes larger,the solvent was determined being a poor solvent, and if it was almostthe same or smaller, a good solvent. The present inventors found outthat the above definitions have extremely good consistency with theeffects of the present invention.

More specifically, whether a solvent is a good solvent or a poor solventto a specific water-insoluble coloring material was determined asfollows. First, the following two water-insoluble coloring materialdispersion solutions A and B were prepared:

A: a water-insoluble coloring material dispersion solution containing50% by mass of a water-soluble organic solvent to be determined, 5% bymass of a water-insoluble coloring material or a total of awater-insoluble coloring material and a substance for dispersingthereof, and 45% by mass of water; and

B: a water dispersion solution containing 5% by mass in total of thewater-insoluble coloring material and the substance for dispersionthereof, put not the water-soluble organic solvent.

Subsequently, the above dispersion solution A was maintained at 60° C.for 48 hours and then cooled to ordinary temperature. The particle sizeof the dispersion solution A was measured with a rich solution particlesize analyzer (Product name: FPAR-1000; manufactured by OtsukaElectronics Co., Ltd.). Likewise, the particle size of the above waterdispersion solution B was measured with the above rich solution particlesize analyzer. Thereafter, the values of the particle sizes of the abovedispersion solution A and water dispersion solution B are denoted by aparticle size (A) and a particle size (B), respectively, and with thesevalues whether the solvent was a good solvent or a poor solvent wasdetermined in accordance with definitions described below. And, an inkhaving the structure of the present invention was prepared using thethus determined good solvent and poor solvent to confirm that theabove-described excellent effects were obtained. Regarding the criteriafor good solvent and poor solvent, when the particle size (A) is largerthan the particle size (B) in the above descriptions, the water-solubleorganic solvent is called a poor solvent. When the particle size (A) isthe same as or smaller than the particle size (B), the water-solubleorganic solvent is called a good solvent.

The water-based ink of the present invention may have the samecomposition as that of the conventional water-based ink containing awater-insoluble coloring material with the exception that water-solubleorganic solvents have the above-described specific features. That is tosay, the first feature of the water-based ink of the present inventionis that it comprises water, a plurality of water-soluble organicsolvents, and a water-insoluble coloring material, wherein thewater-soluble organic solvents include at least one good solvent and atleast one poor solvent that are determined by the above-describeddetermination method, and when Ka values of these water-soluble organicsolvents are determined by the Bristow method and the obtained valuesare compared to each other, the water-soluble organic solvent having thelargest Ka value is a poor solvent. As a consequence, dispersionstability of the water-insoluble coloring material in the ink becomesextremely excellent, and at the same time. it has a sufficiently largearea factor even with a small ink droplet and exhibits a high printdensity when printed on a recording medium, particularly on plain paper,enabling formation of an image with an extremely excellent printquality.

Next, the Ka value determined by the Bristow method is explained. Thisvalue is used as an index of permeability of an ink into a recordingmedium. Taking the case of a liquid ink as an example, when thepermeability of the ink is represented by V, an ink amount per 1 m², theamount of the ink permeated the recording-medium V (mL/m²=μm) after acertain time t has passed after the ejection of an ink droplet isrepresented by the following Bristow's formula:V=Vr+Ka (t−tw)^(½)

Herein, immediately after the ink droplet is applied to the surface ofthe recording medium, almost all of the ink is absorbed in -unevenportions on the surface of the recording medium (rough portions on thesurface of the recording medium), and almost no ink permeates theinternal part of the recording medium. The time when almost no inkpermeates the internal part of the recording medium is referred to as acontact time (tw), and the amount of ink absorbed in uneven portions onthe recording medium during the contact time is referred to as Vr. Whenthe time elapsing after attachment of the ink exceeds the contact time,the amount of the ink permeated the recording medium increases with theextra time exceeding the contact time, that is, by the amount that isproportional to (t−tw)^(½). Ka represents a proportionality constant ofthis increased amount, and it indicates a value corresponding to thepermeation rate. The Ka value can be measured by the Bristow method,using a test set for dynamic permeability of fluids (for example,Product name: Dynamic Permeability Tester S; manufactured by Toyo SeikiSeisaku-sho, Ltd.) or the like.

Moreover, when the total amount (% by mass) of the good solvent in thewater-based ink of the present invention is represented by A and thetotal amount (% by mass) of the poor solvent in the ink is representedby B, the water-based ink of the present invention is adjusted such thatthe ratio of A to B [the total amount (% by mass) of the good solvent inthe ink to the total amount (% by mass) of the poor solvent in the ink]is set within the range between 10:5 or more and 10:30 or less. It is tobe noted that the term “total amount” is used herein to mean that whenplural good solvents exist, for example, all of the good solvents areincluded. In addition, the expression “the ratio of A to B is between10:5 or more and 10:30 or less” is used herein to mean that when A is10, B is between 5 and 30 both inclusive.

Furthermore, in another aspect of the present invention, there isprovided a water-based ink comprising water, a plurality of differentwater-soluble organic solvents, and a water-insoluble coloring material,wherein the above water-soluble organic solvents consist of a goodsolvent to the above water-insoluble coloring material and a poorsolvent to the above water-insoluble coloring material, and when Kavalues of the above water-soluble organic solvents are determined by theBristow method and the obtained values are compared with each other, awater-soluble organic solvent having the largest Ka value is the poorsolvent, and further wherein the adhesive behavior of the ink to a plainpaper is as described below.

It was found that when the water-based ink of the present inventionhaving the above structure was fixed on a plain paper, it showed abehavior that was different from the conventional ink. Such a behaviorenables the ink to have a sufficiently large area factor even with asmall amount of ink droplets and to achieve the formats on of an imagewith high OD (reflection density) Moreover, when a color image in whichdifferent color regions are adjacent to one another is recorded on aplain paper, an effect of preventing feathering can be obtained. Inother words, whether or not it is the ink of the present inventionproviding the above remarkable effects can be determined by measuringthe behavior of the ink when the ink fixed on a plain paper according tothe following method.

In order to measure the behavior of a water-based ink towards a plainpaper, first, a needle with a diameter of 28 G (inner diameter: 0.18 mm,and outer diameter: 0.36 mm) is used, and the tip of the needle islocated at a height of 4 mm from the surface of a plain paper and theink is then dropped from the height. Then, the ink is fixed on thesurface of the paper. While, the diameter of an ink dot is measuredimmediately after the ink landed onto the plain paper and the measuredvalue is represented by dI, and the largest diameter of the spread ofthe ink after the ink is fixed on the plain paper is measured, and themeasured value S represented by dS. Also the largest diameter of thespread of the water-insoluble coloring material in the ink after the inkfixed on the plain paper is measured, and the measured value isrepresented by dC. In the case of the water-based ink of the presentinvention, the above obtained measurement values are in a relationshipof dC<dI<dS (Formula 1). In addition, the penetration depth of thewater-insoluble coloring material into the plain paper is less than 30μm after the ink is printed and fixed on the plain paper by ink netrecording.

The above Formula 1 means that the largest diameter dC of the spread ofthe water-insoluble coloring material in the ink shown in FIG. 14C issmaller than the diameter dI of the ink dot measured immediately afterthe ink landed, which is shown in FIG. 14B, and the largest diameter dSof the spread of the ink after the ink fixed on the recording mediumshown in FIG. 14D) is larger than dI.

Taking into consideration the structure of the Water-based ink of thepresent invention, the above relational expression means that after anink droplet is applied onto a recording medium, owing to the poorsolvent with a high Ka value, the coloring material diffuses and fixesin the vicinity of the surface of the recording medium in the form ofsubstantially a perfect circle, and that the water and water-solubleorganic solvents contained in the ink further diffuse radially in thevicinity of the surface of the recording medium, and that thewater-insoluble coloring material is first immobilized and then thewater and water-soluble organic solvents diffuses into the recordingmedium. Moreover, a feature that the penetration depth of thewater-insoluble coloring material after the Ink fixed on the recordingmedium is less than 30 μm means that when the water-insoluble coloringmaterial is applied to the recording medium, it effectively covers thesurface of the recording medium.

In contrast, when the behavior of a conventional water-based ink to aplain paper is measured in the same manner, the largest diameter dC ofthe spread of the water-insoluble coloring material in the ink after theink fixed on a plain paper becomes greater than the diameter dI of theink dot measured immediately after the ink landed on a plain paper, andthe relationship thereof is expressed as dI<dC<dS. Moreover, the borderbetween the region of water-insoluble coloring material and that of thewater and water-soluble organic solvents spreading therearound becomesunclearer than that with the water-based ink of the present invention.This shows that with a conventional, ink when the water andwater-soluble organic solvents diffuse in the recording medium after theink landed on a plain taper, the water-insoluble coloring material alsospreads together with them, thereby the coloring material cannoteffectively cover the paper surface. Moreover, it also shows that sincethe outline of the coloring material is not a perfect circle, printingalso becomes unclear. Furthermore, even when the relationship dC<dI<dScan hold among dC, dI, and the largest diameter dS of the spread of theink after the ink fixed on the recording medium, if the penetrationdepth of the water-insoluble coloring material after the ink fixation is30 μm or greater, the water-insoluble coloring material does not onlydiffuse in the vicinity of the surface of the paper, but also permeatesin a depth direction of the paper. This case also shows that thecoloring material does not effectively cover the paper.

In the present invention, dI, dS and dC as defined above are measuredaccording to the following methods.

First, a small amount of a water-soluble dye, which is soluble inwater-soluble organic solvents contained in the ink to be measured andhas a hue different from that of the water-insoluble coloring materialbeing a constituent of the ink, was added to the ink, and using the thusobtained ink, the largest diameter dS of the spread of the ink after theink was fixed on the plain paper could be measured by visualobservation. That is to say, by adding a small amount of a water-solubledye, which is soluble in water-soluble organic solvents contained in anink to be measured and has a hue different from that of thewater-insoluble coloring material being a constituent of the ink, thespreading condition of water and water-soluble organic solvents thatdiffuse in a plain paper after the water-insoluble coloring material inthe ink fixed can be confirmed by visual observation cue to the presenceof the above water-soluble dye added to the ink.

The diameter dI of the ink dot immediately after the ink droplet landedon a plain paper was measured using Face CONTACT-ANGLENETER CA-Pmanufactured by Kyowa Interface Science Co., Ltd. More specifically,using a needle with a needle diameter of 28 G (inner diameter: 0.18 mm,and outer diameter: 0.36 mm), the tip of the needle was located a heightof 4 mm from the surface of the plain paper, ink was then dropped fromthe height onto the plain paper, and after the dropping, the diameter ofan ink droplet was read from the scale of the contact anglemeter,thereby measuring the value of dI. That the above read value representsthe ink dot diameter dI immediately after the ink impacts a plain paper.

The largest diameter dS of the spread of the ink after the ink fixed ona plain paper, and, the largest diameter dC of the spread of thewater-insoluble coloring rnadterial in the ink after the ink fixed on aplain paper, were measured as follows. The ink dot dropped on a plainpaper under the above-described conditions was left for 6 hours or more,and after the ink droplet was stabilized, the dimension of the ink dotwas measured. The largest diameter dS of the spread of the ink after theink was fixed on a plain paper was obtained by measuring the largestlinear dimension of the spread hue of a water-soluble dye having a huedifferent from that of the water-insoluble coloring material containedAn the ink. Still further, the largest diameter dC of the spread of thewater-insoluble coloring material in the ink after the ink was fixed ona plain paper was obtained by measuring the largest linear dimension ofthe spread having the hue of the water-insoluble coloring materialcontained in the ink.

Still further, penetration depth of the water-insoluble coloringmaterial after the ink was fixed on a plain paper was obtained bycross-sectioning the printing area of the plain paper after printingwith an ink jet printer and then observing the cross section with amicroscope.

In order that an ink dot has such a form, when the total amount (% bymass) of the good solvent (s) in the ink is denoted by A and the totalamount (% by mass) of the poor solvent(s) En the ink is denoted by B,the ratio of the ratio of A to B [the total amount (% by mass) of thegood solvent(s) in the ink: the total amount (% by mass) of the poorsolvent(s) in the ink] is preferably within the range between 10:5 ormore and 10:30 or less, more preferably within the range between 10:5 ormore and 10:10 or less, and particularly preferably within the rangebetween 10:6 or more and 10:10 or less.

The water-based ink of the present invention is characterized in thatwhen Ka values of the plurality of different types of water-solubleorganic solvents contained in the ink are determined by the Bristowmethod and the obtained values are compared with each other, thewater-soluble organic solvent having the largest Ka value is a poorsolvent. Moreover, according to studies of the present inventors, inorder to achieve further improvement of the quality or the recordedimage, the ink is preferably adjusted such that the Ka value becomesless than 1.5 (ml/m²/msec^(½)), more preferably 0.2 (ml/m²/msec/^(½)) ormore but less than 1.5 (mi/m²/msec^(½)). That is to say, if the ink isconstituted to have a Ka value less than 1.5 (ml/m²/msec^(½)),solid-liquid separation takes place at an early stage of the processwhere the ink permeates a recording medium, so that a high-quality imagewith very little feathering can be formed. At the same time, byadjusting the Ka value of the ink to 0.2 (ml/m²/msec^(½)) or more, morepreferable ability of fixation can be obtained.

It is noted that the Ka value determined by the Bristow method in thepresent invention was measured using, as a recording medium, a plainpaper [e.g., PB paper used for an electrophotographic copying machine,page printer (laser beam printer) or ink jet printer manufactured byCanon Inc., or PPC paper used for an electrophotographic copyingmachine]. The measuring environment was designed assuming an ordinaryoffice environment such as a temperature between 20° C. and 25° C. and ahumidity between 40% and 60%.

By the way, when an image is formed with both black and color inks on aplain paper, if the water-based ink of the present invention is used asa black ink as described above, it is considered that the agglomerationor disruption of the dispersion of the coloring material constitutingthe black ink progresses faster than the case of other inks. In theimage forming method of the present invention, the water-based ink ofthe present invention is used as a black ink, and the formation of animage with a color ink is carried out after the formation of an imagewith the black ink, and more preferably, scanning for attaching theblack ink is carried out, and after at least an interval of a singlescanning, scanning for attaching the color ink is carried out. Byadopting such a process, even when the black ink comes into contact withthe color ink, color mixing or bleeding between the black ink and thecolor ink does not occur on the paper, thereby achieving excellentanti-bleeding in the print. That is to say, according to the presentinvention, the above-described excellent effects can be achieved by onlycarrying out image formation with each of black and color inks with acertain time interval, dispensing with a multipass printing method ofplural scanning which needs a long printing time, or a method of usingdifferent recovery systems for black and color inks which leading toup-sizing in the apparatuses.

Moreover, when the water-based ink of the present invention is used,since the coloring material contained in the ink efficiently remains ona recording medium for the reasons described above, it becomes possibleto conduct high-density printing with a small ejected amount of the ink(droplet volume) than that of the conventional ink Furthermore, sinceprinting can be conducted with a smaller amount of ink, effects such ascost reduction in image formation or achievement of a faster fixationtime in comparison with the conventional ink, can be obtained.

The water-based ink of the present invention is characterized in thatthe water-soluble organic solvents contained in the ink are theconstitution as described above in the relation with the water-insolublecoloring material to be used. Other than such a constitution, thewater-based ink of the present invention may adopt the same constitutionas that of the conventional water-based ink. Each component comprisingthe ink of the present invention will be described below. First, theaqueous medium dispersing the water-insoluble coloring material will beexplained.

<Aqueous Medium>

The water-based ink of the present invention comprises a mixed solventof water-and water-soluble organic solvents. The water-soluble organicsolvents can be selected from the items listed below. In the presentinvention, when the water-soluble organic solvents are selected, it isfirst determined whether the solvent is a good solvent or a poor solventto the water-insoluble coloring material to be used, and then, based onthe determination results, the water-soluble organic solvents areselected such that at least both a good solvent and a poor solvent arecontained and that the content of each water-soluble organic solvent iswithin the range defined in the present invention, and they are thenappropriately blended, so as to prepare an ink.

Specific examples of such water-soluble organic solvents may include:alkyl alcohols containing 1 to 4 carbon atoms such as methyl alcohol,ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol,sec-butyl alcohol or tert-butyl alcohol; amides such asdimethylformamide or dimethylacetamide; ketones or keto alcohols such asacetone or diacetone alcohol; ethers such as tetrahydrofuran or dioxane;polyalkylene glycols such as polyethylene glycol or polypropyleneglycol; alkylene glycols whose alkylene group contains 2 to 6 carbonatoms, such as ethylene glycol, propylene glycol, butylene glycol,triethylene glycol, 1,2,6-hexanetriol, thiodiglycol, hexylene glycol ordiethylene glycol; lower alkyl ether acetate such as polyethylene glycolmonomethyl ether acetate; glycerin; lower alkyl ethers of polyalcohols,such as ethylene glycol monomethyl (or ethyl) ether, diethylene glycolmethyl (or ethyl) ether, or triethylene glycol monomethyl (or ethyl)ether; N-methyl-2-pyrrolidone, 2-pyrrolidone, and1,3-dimethyl-2-imidazolidinone. Moreover, deionized water is preferablyused as water.

The content of the water-soluble organic solvents in the water-based inkof the present invention is not particularly limited. It is preferablywithin the range between 3% by mass and 50% by mass based on the totalmass of the ink. The content of water in the ink is preferably withinthe range between 50% by mass and 95% by mass based on the total mass ofthe ink.

In a preferred embodiment of the present invention, the type and contentof the water-soluble organic solvents constituting the water-based inkare adjusted such that when the total amount (V by mass) of a goodsolvent (s) in the ink is denoted by A and the total amount (% by mass)of a poor solvent(s) in the ink is denoted by B, the ratio of the ratioof A to B ranges from 10:5 to 10:30 both inclusive, preferably withinthe range between 10:5 or more and 10:10 or less, and more preferablywithin the range between 10:6 or more and 10:10 or less.

According to detail-ed studies of the present inventors, when the ratioof the good solvent(s) contained in the water-based ink is higher thanthe above range, it becomes difficult to obtain a high print density,although it is excellent in storage stability. In contrast, when theratio of the good solvent(s) contained in the water-based ink is lowerthan the above range, sufficient storage stability might not beobtained, although a high print density can be obtained. Contrary tothese cases, when the ratio of the good solvent(s) and the poorsolvent(s) in the water-soluble organic solvents contained in the ink isadjusted as above, both the storage stability of the ink and a highprint density can be obtained. In addition, in the present invention, asstated above, when the type of the water-soluble organic solventscontained in the ink is determined according to the Ka value determinedby the Bristow method, which is an index indicating the permeability ofeach water-soluble organic solvent into a recording medium, awater-based ink can be obtained tha0t has a sufficiently large areafactor even with a small amounts of ink droplet and can achieve a highprint density. Such effects have not been achieved so far.

The storage stability of an ink generally means the stability of an inkin the state with no evaporation of water. The high print density isrealized by the agglomeration phenomenon of the pigment, which occurswith the spread of the solvent on a paper during the ink dot formationwhen the ink landed on the paper.

Moreover, other than the above-described effects such as realization ofboth storage stability and high print density, the combined use of agood solvent and a poor solvent in the ink brings about an effect ofpreventing the agglomeration of the pigment to a certain extent when thewater evaporates from the ink. More specifically, when ink droplets areattached onto the nozzle face of a recording head, the density of acoloring material in the ink is increased by evaporation of the ink. Atthis time, coexistence of the good solvent and the poor solvent in theink can prevent the agglomeration of the pigment to a certain extent bythe action of the good solvent. Thus, when a good solvent is containedin the ink, the agglomeration of the pigment on the nozzle face of arecording head is prevented, and thereby it can also be expected thatreliability in the recovery operation of the recording head will beenhanced.

<Water-Insoluble Coloring Material>

A water-insoluble coloring material constituting the water-based ink ofthe present invention will now be explained. Regardless of itsdispersion system, the water-insoluble coloring material constitutingthe water-based ink of the present invention may be a pigment dispersedwith a resin dispersant or surfactant (a resin-dispersed pigment,surfactant-dispersed pigment), or a coloring material dispersiblewithout using a dispersant etc. owing to the high dispersibility of thecoloring material itself such as a microencapsulated pigment, aself-dispersing pigment having hydrophilic groups attached onto thesurface or the pigment particles and a modified pigment in whichpolymeric organic groups are chemically bonded to the surface or pigmentparticles (polymer-binding self-dispersing pigment). Naturally, thesepigments dispersed of different types may be used in combination. Thecontent of the water-insoluble coloring material is 0.1% to 15% by mass,and more preferably 1% to 10; by mass based on the total mass of theink. These pigments used in the present invention will be explainedbelow.

[Pigment]

Pigments used for the water-based ink of the present invention are notparticularly limited, and any of the following pigments can be used.

Carbon black is preferable as a pigment used for black ink. Examples ofsuch carbon black may include furnace black, lamp black, acetylene blackand channel black. More specifically, commercially available productsmentioned below can be used: Raven 7000, Raven 5750, Raven 5250, Raven5000 ULTRA, Raven 3500, Raven 2000, Raven 1500, Raven 1250, Raven 1200,Raven 1190 ULTRA-II, Raven 1170, and Raven 1255(all of which aremanufactured by Colombia Co., Ltd.), Black Pearls L, Regal 400R, Regal330R, Regal 660R, Mogul L, Monarch 700, Monarch 800, Monarch 880,Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, Monarch 1400,Monarch 2000, and Valcan XC-72R (all of which are manufactured by CabotCorp.), Color Black FW1, Color Black FW2, Color Black FW2V, Color BlackFW18, Color Black FW200, Color Black S150, Color Black S160, Color BlackS-170, Printex 35, Printex U, Printex V, Printex 140U, Printex 140V,Special Black 6, Special Black 5, Special Black 4A, and Special Black 4(all of which are manufactured by Degussa Corp.), No. 25, No. 33, No.40, No. 47, No. 52, No. 900, No. 2300, MCF-88, MA600, MA7, MA8, andMA100 (all of which are manufactured by Mitsubishi Chemical Corp.)Further, carbon black, which has been newly produced for the presentinvention, can also be used. However, the present invention is notlimited thereto, but any of the conventionally known carbon blacks canbe used. Furthermore, such a black pigment is not limited to carbonblack, but magnetic particles such as magnetite or ferrite, or titaniumblack may also be used as such a black pigment.

Specific examples of an organic pigment may include insoluble azopigments such as Toluidine Red, Toluidine Maroon, Hansa Yellow,Benzidine Yellow or Pyrazolone Red, soluble azo pigments such as LitholRed, Helio Bordeaux, Pigment Scarlet or Permanent Red 2B, derivativesfrom vat dyes such as alizarin, indanthrone or thioindigo maroon,phthalocyanine pigments, such as phthalocyanine blue or phthalocyaninegreen, quinacridone pigments such as quinacridone red or quinacridonemagenta, perylene pigments such as perylene red or perylene scarlet,isoindolinone pigments such as isoindolinone yellow or isoindolinoneorange, imidazolone pigments such as benzimidazolone yellow,benzimidazolone orange or benzimidazolone red, pyranthrone pigments suchas pyranthrone red or pyranthrone orange, indigo pigments, condensed azopigments, thloindigo pigments, diketopyrrolopyrrole pigments,flavanthrone yellow, acylamide yellow, quinophthalone yellow, nickel azoyellow, copper azo methine yellow, perinone orange, anthrone orange,dianthraquinonyl red, and dioxazine violet. Naturally, organic pigmentsare not limited thereto, but other organic pigments may also be used.

Moreover, when organic pigments that can be used in the presentinvention are expressed by the color index (C. I.) number, examples ofsuch C. I. numbers may include C. I. pigment yellow Nos. 12, 13, 14, 17,20, 24, 74, 83, 86, 93, 97, 109, 110, 117, 120, 125, 128, 137, 138, 147,148, 150, 151, 1.53, 154, 166, 168, 180 and 185, C. I. pigment orangeNos. 16, 36, 43, 51, 55, 59, 61 and 71, C. I. pigment red Nos. 9, 48,49, 52, 53, 57, 97, 122, 123, 149, 168, 175, 176, 177, 180, 192, 215,216, 217, 220, 223, 224, 226, 227, 228, 238, 240, 254, 255 and 272, C.I. pigment violet Nos. 19, 23, 29, 30, 37, 40 and 50, C. I. pigment blueNos. 15, 15: 1, 15: 3, 15: 4, 15: 6, 22, 60 and 64, C. I. pigment greenNos. 7 and 36, and C. I. pigment brown Nos. 23, 25 and 26.

[Resin-Dispersed Pigment]

As stated above, a resin-dispersed pigment, i.e., a pigment dispersibleby using a dispersant, can be used as a water-insoluble coloringmaterial contained in the water-based ink of the present invention. Inthis case, a compound for dispersing the above-listed hydrophobicpigment is required. As such compounds, so-called dispersants includingsurfactants and resin dispersants or the like can be used. Suchdispersants or surfactants are not particularly limited, but amongothers, anionic compounds or nonionic compounds can be preferably usersExamples of such an anionic compound may include a-fatty acid salt,alkyl sulfate, alkylbenzene sulfonate, alkylnaphthalene sulfonate,dialkyl sulfosuccinate, alkyl phosphate, formalin condensates ofnaphthalenesulfonate, alkyl polyoxyethylene sulfate, and substitutedderivatives thereof. Examples of such a nonionic compound may includepolyoxyethylene alkyl ether, polyoxyethylene alkyl phenyl ether,polyoxyethylene fatty acid ester, sorbitan fatty acid ester,polyoxyethylene sorbitan fatty acid ester, polyoxyethylene alkylamine,glycerin fatty acid ester, an oxyethyleneoxypropylene block polymer, andsubstituted derivatives thereof. Examples of a resin dispersant mayinclude a block copolymer, a random copolymer, a graft copolymer, andsalts thereof, which consist of at least two monomers at least one ofthem being a hydrophilic monomer) selected from the group consisting ofstyrene and derivatives thereof, vinylnaphthalene and derivativesthereof, fatty acid alcohol esters of α,β-ethylene unsaturatedcarboxylic acid, acrylic acid and derivatives thereof, maleic acid andderivatives thereof, itaconic acid and derivatives thereof, fumaric acidand derivatives thereof, and vinyl acetate, vinyl alcohol,vinylpyrrolidone, acrylamide and their derivatives.

[Microencapsulated Pigment]

As stated above, a water-insoluble coloring material may be coated withan organic polymer for microencapsulation so as to obtain amicroencapsulated pigment, which can be used as a water-insolublecoloring material for the water-based ink of the present invention.Examples of such a method of covering a water-insoluble coloringmaterial with organic polymers for microencapsulation may include achemical production method, a physical production method, aphysico-chemical production method, and a mechanical production method.Specific examples of such methods may include interfacial polymerizationmethod, in-situ polymerization method, liquid-submerged hardeningcoating method, coacervation (phase separation) method, liquid-submergeddrying method, melting dispersion cooling method, air suspension coatingmethod, spray drying method, acid deposition method, and phase inversionemulsification method.

Examples of an organic polymer used as a material for a microcapsule mayinclude polyamide, polyurethane, polyester, polyurea, epoxy resin,polycarbonate, urea resin, melamine resin, phenol resin, polysaccharide,gelatin, gum Arabic, dextran, casein, protein, natural rubber,carboxypolymethylene, polyvinyl alcohol, polyvinylpyrrolidone, polyvinylacetate, polyvinyl chloride, polyvinylidene chloride, cellulose, ethylcellulose, methyl cellulose, nitrocellulose, hydroxyethyl cellulose,cellulose acetate, polyethylene, polystyrene, a polymer or copolymer of(meth)acrylic acid, a polymer or copolymer of (meth)acrylic acid ester,a (meth)acrylic acid-(meth)acrylic acid ester copolymer, astyrene-(meth)acrylic acid copolymer, a styrene-maleic acid copolymer,soda alginate, fatty acid, paraffin, beeswax, Chinese wax, solid beeftallow, carnauba wax, and albumin.

Among these, organic polymers having an anionic group such as acarboxylic acid group or sulfonic acid group can be used. In addition,examples of a nonionic organic polymer may include polyvinyl alcohol,polyethylene glycol monomethacrylate, polypropylene glycolmonomethacrylate, methoxy polyethylene glycol monomethacrylate,(co)polymers thereof, and cationic ring-opening polymers of 2-oxazoline.Of these, a completely ketonized product of polyvinyl alcohol isparticularly preferable because it has low water solubility and is easyto dissolve in hot water but difficult to dissolve in told water.

When the phase separation method or the acid deposition method isselected as a microencapsulation method, anionic organic polymers can beused as the organic polymer to form microcapsules. The phase separationmethod uses, as an organic solvent phase, a composite or complexconsisting of an anionic organic polymer having an ability forself-dispersing or dissolving in water and a coloring material such as aself-dispersing organic pigment or carbon black, or a mixture consistingof a coloring material such as a self-dispersing organic pigment orcarbon black, a hardening agent, and anionic organic polymers The phaseseparation method involves adding water to the above organic solventphase, or adding the above organic solvent phase into water, so as toconduct microencapsulation with self-dispersion (phase separationemulsification). In the above phase separation method, water-solubleorganic solvents or additives to be used for the ink may be added intothe organic solvent phase. Considering that a dispersion solution forthe ink can be directly produced, it is particularly preferable to mix aliquid medium for the ink into the organic solvent phase.

On the other hand, in the acid deposition method, a hydrous cake isobtained by the steps of: neutralizing a part or all of the anionicgroups of anionic group-containing organic polymer with a basic compoundand kneading the polymer with a coloring material such as aself-dispersing organic pigment or carbon black in an aqueous medium;and adjusting the pH of the mixture to neutral or acidic with an acidcompound so that the anionic group-containing organic polymerprecipitates to stick to the pigment. The obtained hydrous cake issubjected to microencapsulation by neutralizing a part or all of theanionic groups with a basic compound. By this method, an anionicmicroencapsulated pigment, which is fine and containing a large amountof pigments, can be produced

Examples of a solvent used in the above microencapsulation may includealkyl alcohols such as methanol, ethanol, propanol or butanol; aromatichydrocarbons such as benzol, toluol or xylol; esters such as methylacetate, ethyl acetate or butyl acetate; chlorinated hydrocarbons suchas chloroform or ethylene dichloride; ketones such as acetone or methylisobutyl ketone; ethers such as tetrahydrofuran or dioxane; andcellosolves such as methyl cellosolve or butyl cellosolve. Moreover, themicrocapsules produced by the above-described method are subjected tocentrifugal separation or filtration to separate it from the solvent,they are then mixed with water or necessary solvents, and the mixture isstirred and then dispersed again, so as to obtain a micro-encapsulatedpigment of interest. The mean particle size of the microencapsulatedpigment obtained by the above method is preferably between 50 nm and 180nm.

[Self-Dispersing Pigment]

As stated above, as a water-insoluble coloring material contained in thewater-based ink of the present invention, a self-dispersing pigmentcapable of dispersing without a dispersant can be used. As aboveself-dispersing pigment, there is a pigment in which hydrophilic groupsare chemically bonded to the surface of pigment particles, eitherdirectly or via another group of atoms. For example, pigments in which ahydrophilic group introduced to the surface of a pigment particle is oneselected from the group consisting of -COOM1, —SO₃M-1, and —PO₃H-(M1)₂(wherein MI represents any one selected from the group consisting of ahydrogen atom, an alkali metal, ammonium, and organic ammonium) can bepreferably used. The above-mentioned another atom group may be oneselected from the group consisting of an alkylene group containing to 12carbon atoms, a substituted or unsubstituted phenylene group, and asubstituted or unsubstituted naphthylene group, can also be preferablyused. In addition, self-dispersing pigments obtained by the followingsurface oxidization methods can also be preferably used: a method ofoxidizing carbon black with sodium hypochlorite; a method of oxidizingcarbon black by ozonation in water; and a method comprising subjectingcarbon black to ozonation and then subjecting it to wet oxidation withan oxidizing agent, so as to modify the surface of the carbon black.

[Polymer-Binding Self-Dispersing Pigment]

As stated above, as a water-insoluble coloring material, contained inthe water-based ink of the present invention, a polymer-binding typeself-dispersing pigment capable of dispersing without using dispersantscan be used. Such a polymer-binding self-dispersing pigment using nodispersant is preferably a reaction product between functional groupsthat are chemically bonded to the surface of the pigment, eitherdirectly or via another group of atoms, and a copolymer of an ionicmonomer and a hydrophobic monomer. That is to say, when thepolymer-binding type self-dispersing pigment having such a structure isused, the copolymerization ratio between the ionic monomer and thehydrophobic monomer, raw materials of the copolymer to modify thesurface of the pigment, can be changed property, thereby enabling propercontrol of the hydrophilicity of the modified pigment. Accordingly, thepolymer-binding type self-dispersing pigment with the above structureSkis preferable. Moreover, the type of an ionic monomer and ahydrophobic monomer to be used can be selected as appropriate, or thecombination thereof can be changed as appropriate, so that variousproperties can be added to the surface of the pigment. From this pointalso, the above polymer-binding type self-dispersing pigment ispreferable.

(Functional Group)

Functional group contained in the above polymer-binding typeself-dispersing pigment is chemically bonded to the surface of thepigment, directly or via another atom group. The functional groupconstitutes an organic group by a reaction with a copolymer describedlater. The type of the functional group is selected herein inassociation with the functional, group of the copolymer. Taking intoconsideration the fact that the pigment is dispersed in an aqueousmedium, the reaction between the functional group and the copolymerpreferably generates a bond which is not hydrolyzed, such as a reactioncausing an amide bond. An amino group is used, as the functional group,and the copolymer has a carboxyl group, so that the copolymer can beintroduced to the surface of a pigment particle via an amide bond.Otherwise, a carboxyl group is used as the functional group, and thecopolymer supports an amino group, so that the copolymer can beintroduced to the surface of a pigment particle via an amide bond.

Herein, the functional group that is chemically bonded to the surface ofa pigment may be directly bonded thereto, or may be bonded thereto viaanother atom group. However, when a copolymer with a relatively highmolecular weight is introduced to the surface of a pigment, in order toprevent steric hindrance among copolymers, it is preferable to introducethe functional group into the surface of a pigment via another atomgroup. Another atom group is not particularly limited herein as long asit is a multivalent element or organic group. From the viewpoint ofadjusting the distance of the functional group from the surface of apigment for the above-described reasons, for example, a divalent organicresidue is preferably used. Examples of such a divalent organic residuemay include al alkylene group and an arylene group (phenylene group).

More specifically, in Examples described later for instance, a pigmentis reacted with aminophenyl(-sulfoethyl)sulfone, so as to introduce aphenyl (2-sulfoethyl)sulfone group into the surface of the pigment.Thereafter, an amino group of pentaethylenehexamine is reacted with thephenyl(2-sulfoethyl)sulfone group, so as to introduce an amino group asa functional group. In this case, the amino group is chemically bondedto the surface of a pigment via an atom group containing aphenyl(2-sulfoethyl) group.

(Copolymer of Polymer-Binding Type Self-Dispersing Pigment)

An anionic copolymer having anionic properties or a cationic copolymerhaving cationic properties is preferably used as the above-describedcopolymer consisting of an ionic monomer and a hydrophobic monomer.

Examples of the above anionic copolymer may include a copolymerconsisting of a hydrophobic monomer and an anionic monomer, and saltsthereof. Representative hydrophobic monomers used in the abovecopolymerization may include, but not limited to, methacrylic acid alkylesters such as styrene, vinylnaphthalene or methyl methacrylate, acrylicacid alkyl esters such as phenyl methacrylate, benzyl methacrylate,2-ethoxyethyl methacrylate, methacrylonitrile, 2-trimethylsiloxyethyl.methacrylate, glicidyl methacrylate, p-tolyl methacrylate, sorbylmethacrylate or methyl acrylate, phenyl acrylate, benzyl acrylate,acrylonitrile, 2-trimethylsiloxyethyl acrylate, glycidyl acrylate,p-tolyl acrylate, and sorbyl acrylate.

Examples of an anionic monomer used in the above copolymerization mayinclude but not limited to acrylic acid, methacrylic acid, and maleicacid.

In an embodiment of the copolymer used in the present invention, ananionic copolymer consisting of an anionic monomer and a hydrophobicmonomer consists of at least two monomers, which consist of any oneselected from the above listed hydrophobic monomers and at least oneselected from the above listed anionic monomers. This copolymer includesa block copolymer, a random copolymer, a graft copolymer, and saltsthereof.

The acid value of the anionic copolymer is preferably within the rangebetween 100 and 500. Moreover, an anionic copolymer wherein thevariation in the acid values is 20%t or less of the mean acid value ispreferably used. By setting the acid value of the copolymer within theabove range, a problem that the hydrophilicity of the pigment surface isso high that water and solvents contained in the ink stay on the pigmentsurface after printing, thereby causing slow expression of the markerresistance of the ink after printing on a recording medium, can beeffectively reduced. Moreover, another problem that excessively lowhydrophilicity of the-pigment surface prevents the pigment from beingstably dispersed in the ink can also be effectively reduced.

Examples of the above salts may include alkali metal salts such assodium, lithium or potassium, ammonium salts, alkylamine salts, andalkanolamine salts. These salts can be used as appropriate, singly or incombination of several types.

Next, in another embodiment of the copolymer used in the presentinvention, a cationic copolymer consisting of a cationic monomer and ahydrophobic monomer will be explained. Examples of such a cationiccopolymer may include copolymers consisting of hydrophobic monomers andcationic monomers listed below, or salts thereof. As hydrophobicmonomers, the above listed monomers can be used.

Examples of a cationic monomer used herein may include allylamine,dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate,tertiary-butylaminoethyl methacrylate, dimethylaminoethyl acrylate,diethylaminoethyl acrylate, dimethylaminoethyl metacrylamide,N-vinylcarbazole, methacrylamide, acrylamide, and dimethylacrylamide.

Examples of a cationic copolymer may include a block copolymer, a randomcopolymer, and a graft copolymer, which consist of at least two monomerscontaining a hydrophobic monomer and a cationic monomer selected fromthe above monomers, and salts thereof. In particular, cationiccopolymers having an amine value within the range between 100 and 500are preferable. In addition, the variation in the amine values ispreferably 20% of less of the mean amine value. The amine value isdenoted by the mg number of KOH that is equivalent to hydrochloric acidnecessary to neutralize 1 g of a sample. Examples of the above salts mayinclude acetic acid, hydrochloric acid, and nitric acid. These salts canbe used as appropriate, singly or in combination of several types.

The weight-average molecular weight (MW) of the above-described anionicor cationic copolymer is preferably within the range between 1,000 and20,000, and more preferably within the range between 3,000 and 20,000.Moreover, those wherein the polydispersity Mw/Mn (weight-averagemolecular weight MW/number-average molecular weight Mn) of a cationiccopolymer segment is 3 or less are preferably used. The content of sucha cationic copolymer in the ink is preferably between 5% by mass and 40%by mass based on the total mass of the pigment particles whose surfaceis modified by the copolymer. With regard to polydispersity of thecopolymer, when the polydispersity is large, it results in a widemolecular-weight distribution of the copolymer, and the aforementionedproperties of the copolymer that are based on the molecular weight arehardly expressed. Accordingly, the molecular-weight distribution of thecopolymer is preferably uniform.

Next, taking carbon black as an example, a method for modifying apigment by chemically binding an organic group to the surface of apigment particle will be explained. Methods are not particularlylimited, and any commonly used method can be used herein, as long as themethods involve introducing functional groups on the surface of apigment particle, binding a copolymer consisting of an ionic monomer anda hydrophobic monomer to these functional groups, so that the copolymeris chemically bonded to the surface of the pigment particle. Forexample, the following methods can be used.

A method comprising introducing polyethyleneimine or the like into thesurface of a pigment particle such as carbon black and binding acopolymer consisting of an ionic monomer and a hydrophobic monomer andhaving an amino group to its terminal functional group by a diazoniumreaction, or a method of binding a copolymer having an amino group and acarboxyl group in a molecule thereof to the surface of a pigment such ascarbon black by a diazonium reaction, can be applied. Other than thesemethods, The most typical example is disclosed in WO 01/51566 A1.

In the above-described methods, where an anionic copolymer is chemicallybonded to the surface of a carbon black particle for example, thefollowing 3 steps are carried out:

the first step of attaching an aminophenyl(2-sulfoethyl)sulfone group(APSES) to the carbon black by a diazonium reaction;

the second step of attaching polyethyleneimine or pentaethylenehexamine(PEHA) to the above carbon black treated with APSES; and

the third step of binding a. copolymer consisting of a hydrophobicmonomer and an ionic monomer having a carboxyl group to the particlesurface.

In the above second step, the phenyl(2-sulfoehtyl)sulfone groupchemically bonded to the surface of the carbon black by the first stepis reacted with, an amino group of APSES, so that the amino group isintroduced to the surface of the carbon black as a functional group thatis chemically bonded thereto. In the third step, for example, a part ofa carboxyl group contained in the ionic monomer portion of a copolymerIs reacted with an amino group to form an amide linkage, so that thecopolymer is introduced to the surface of the carbon black via an atomgroup containing a phenyl(2-sulfoethyl) group as a residue of APSES anda residue of PEHA.

In addition, in the above-described method, where a cationic copolymeris chemically bonded to the surface of a carbon black particle forexample, the method comprises the following two steps:

the first step of attaching an aminophenyl(2-sulfoethyl)sulfone group(APSES) to the carbon black by a diazonium reaction; and

the second step of binding a copolymer consisting own a hydrophobicmonomer and a cationic monomer to the particle surface. By the abovefirst step, a sulfone group is introduced to the surface of the carbonblack as a functional group chemically bonded thereto. Thereafter, bythe above second step, for example, a part of an amino group containedin the ionic monomer portion of a copolymer is reacted with a sulfonegroup (nucleophilic substitution), so that the copolymer is introducedto the surface of the carbon black via an atom group containing aphenyl(2-sulfoethyl) group as a residue of APSES.

[Water-Soluble Resin-Adsorbing Self-Dispersing Pigment]

As stated above, a resin-dispersed pigment, a microencapsulated pigment,a self-dispersing pigment, and a polymer-binding type self-dispersingpigment can be used as a water-insoluble coloring material contained inthe water-based ink of the present invention. Other than these pigments,a pigment obtained by adding a water-soluble resin to a pigment can alsobe used. Such a water-soluble resin-containing pigment will be explainedbelow.

In the present invention, water-soluble resin-adsorbing self-dispersingpigments obtained by adding a water-soluble resin to the above listedself-dispersing pigments can also be used.

Such water-soluble resin-adsorbing self-dispersing pigments can beproduced by adding a water-soluble resin to the above-describedself-dispersing pigment and stirring and blending the mixture. Thepreferable examples of a water-soluble resin-adsorbing self-dispersingpigment are described below.

A pigment wherein a hydrophilic group is chemically bonded to thesurface of a pigment particle directly or via another atom group is anexample of the self-dispersing pigments used for the water-solubleresin-attached self-dispersing pigments that can be used in the presentinvention. For example, pigments in which a hydrophilic group introducedto the surface of a pigment particle is one selected from the groupconsisting of -COOM1, SO₃M1, and —PO₃H(M1)₂ (whereip M1 represents anyone selected from the group consisting of a hydrogen atom, an alkalimetal, ammonium, and organic ammonium) can be preferably used. The aboveanother atom group, selected from the group consisting of an alkylenegroup containing 1 to 12 carbon atoms, a substituted or unsubstitutedphenylene group, and a substituted or unsubstituted naphthylene group,can also be preferably used.

In addition, self-dispersing pigments obtained by the following surfaceoxidization methods can also be preferably used: a method of oxidizingcarbon black with sodium hypochlorite; a method of oxidizing carbonblack by ozonation in water; and a method comprising subjecting carbonblack to ozonation and then subjecting it to wet oxidation with anoxidizing agent, so as to modify the surface of the carbon black.Alkali-soluble polymers are preferably adsorbed at a certain ratio toself-dispersing pigments used to produce a water-soluble resin-adsorbingself-dispersing pigment. If the surface of a self-dispersing pigment ismodified to the maximum by the above-described carboxylation or thelike, adsorption of alkali-soluble polymer to a pigment particle becomessubstantially zero. In contrast, if the modification of the surface istoo small, effects by the modification are hardly obtained. Accordingly,as an index of the degree of surface modification, the surfacemodification is carried out such that the content of carboxyl groups onthe surface is set within the range between 0.1 and 0.5 mmol/g.

Examples of a water-soluble resin that can be used to produce awater-soluble resin-adsorbing self-dispersing pigment used in thepresent invention may include a copolymer consisting of a hydrophobicmonomer and an anionic monomer, a copolymer consisting of a hydrophobicmonomer, a nonionic monomer and an anionic monomer, and salts thereof.Examples of a representative hydrophobic monomer used herein mayinclude, but not limited to, methacrylic acid alkyl esters such asstyrene, vinylnaphthalene or methyl methacrylate, acrylic acid alkylesters such as phenyl methacrylate, benzyl methacrylate, 2-ethoxyethylmethacrylate, methacrylonitrile, 2-trimethylsiloxyethyl methacrylate,glycidyl methacrylate, p-tolyl methacrylate, sorbyl methacrylate ormethyl acrylate, phenyl acrylate, benzyl acrylate, acrylonitrile,2-trimethylsiloxyethyl acrylate, glycidyl acrylate, p-tolyl acrylate,and sorbyl acrylate. Examples of a nonionic monomer may include but notlimited to hydroxyethy (meth)acrylate, polyethylene glycol(meth)acrylate, alkoxy polyethylene glycol (meth)acrylate, and siliconmacromer. Examples of an anionic monomer used herein may include but notlimited to acrylic acid, methacrylic acid, and maleic acid. Thecopolymer herein includes a block copolymer, a random copolymer, a graftcopolymer, and salts thereof.

The acid value of the anionic copolymer used herein is preferably withinthe range between 100 and 500. Moreover, an anionic copolymer whereinthe variation in the acid values is 20%- or less of the mean acid valueis preferably used. Examples of the above salts may include alkali metalsalts such as sodium, lithium or potassium, ammonium salts, alkylaminesalts, and alkanolamine salts. These salts can be used as appropriate,singly or in combination of several types. The weight-average molecularweight (MW) of the above-described anionic copolymer is preferablywithin the range between 1,000 and 20,000, and more preferably withinthe range between 3,000 and 20,000.

The term adsorption between a pigment and a water-soluble resin is usedin the present invention to mean adsorption caused by van der Waalsforce or intermolecular force. As means for causing adsorption, ordinarystirring of a pigment and an alkali-soluble polymer is sufficient, sluta dispersing device that applies high shearing when the pigment adsorbsthe alkali-soluble polymer may be used. In order to determine the degreeof adsorption of an alkali-soluble polymer to a self-dispersing pigment,evaluation using surface tension is appropriate. For example, surfacetension is measured when a water-soluble resin is gradually added to 1%by mass of self-dispersing pigment (the amount of an alkali-solublepolymer vs. γ in a system containing 19. by mass of self-dispersingpigment: plot A), and surface tension is measured with aqueous solutionsof a water-soluble resin at various concentrations (thee amount of awater-soluble resin vs. γ: plot B). By comparing plot A with plot B todetermine the difference in concentrations of water-soluble resin atwhich a certain surface tension is obtained, it is possible to estimatethe approximate amount of a water-soluble resin adsorbed to the pigment.

<Other Components>

In order to maintain moisture, the water-based ink of the presentinvention may contain moisture retentive solids such as urea, ureaderivatives, trimethylolpropane or trimethylolethane as ink components,other than the above-described components. In general, the content ofmoisture retentive solids such as urea, urea derivatives ortrimethylolpropane in the ink is preferably within the range between0.1% by mass and 20.0% by mass, and more preferably within the rangebetween 3.0% by mass and 10.0% by mass based on the total mass of theink.

In addition, other than the above components, the ink of the presentinvention may also comprise, as necessary, various additives such as asurfactant, pH adjuster, anticorrosive agent, antiseptic agent,fungicide, antioxidant, anti-reduction agent, evaporation-promotingagent or chelating agent.

Preferred examples of a surfactant used in .he present invention mayinclude compounds having any one of the following formulas (1) to (4):Formulas (1)R—O—(CH₂CH₂O)_(n)-H(wherein R represents an alkyl group, and n represents an integer.)

(wherein R represents an alkyl group, and n represents an integer.)

(wherein R represents a hydrogen atom or alkyl group, and each of it mand n represents an integer.)

(wherein each of m and n represents an integer.)

<<Image Forming Method Using the Above Ink>>

The image forming method of the present invention is an ink jetrecording method for recording on a plain paper using a black ink and atleast one water-based color ink. The method is characterized in thatwater-based ink having the above-described structure is used as a blackink, and in that when an image is formed which consists of an imageformed from the black ink adjacent to an image formed from the colorink, the image is formed by scanning for attaching the black ink, andthen scanning for attaching the color ink to a region in which the imagehas been formed.

<Color Inks Used Together>

Now, color inks used together with the black ink in the presentinvention will be explained. In the image forming method of the presentinvention, any conventionally known water-based inks used in ink jetrecording can be used. Coloring materials for color inks may includewater-soluble dyes, and in particular, water-soluble dyes having ananionic group as a solubilizing group are preferable. The color of thecolor ink used in the present invention can be appropriately selectedfrom the group consisting of cyan, magenta, yellow, red, green, blue andorange.

The water-soluble dyes having an anionic group used in the presentinvention are not particularly limited, as long as they arewater-soluble acid dyes, direct dyes, or reactive dyes described in thecolor index. Moreover, although it is not a dye described in the colorindex, if the dye has an anionic group such as a sulfone group, it isavailable. These dyes are used within the range between 1% by mass and10 by mass, and preferably within the range between 1% by mass and 5% bymass based on the total mass of the ink.

Specific examples of such a dye are as follows:

C. I. direct yellow: 8, 11, 12, 27, 28, 33, 39, 44, 50, 58, 85, 86, 87,88, 98, 100, 110

C. I. direct red: 2, 4, 9, 11, 20, 23, 24, 31, 39, 46, 62, 75, 79, 80,83, 89, 95, 197, 201, 218, 220, 224, 226, 227, 228, 230

C. I. direct blue: 1, 15, 22, 25, 41, 76, 77, 80, 86, 90, 98, 106, 108,120, 158, 163, 168, 199, 226

C. I. acid yellow: 1, 3, 7, 11, 17, 23, 25, 29, 36, 38, 40, 42, 44, 76,98, 99

C. I. acid red: 6, 8, 9, 13, 14, 18, 26, 27, 32, 35, 42, 51, 52, 80, 83,87, 89, 92, 94, 106, 114, 115, 133, 134, 145, 158, 198, 249, 265, 289

C. I. acid blue: 1, 7, 9, 15, 22, 23, 25, 29, 40, 43, 59, 62, 74, 78,80, 90, 100, 102, 104, 117, 127, 138, 158, 161

Other than the above listed dyes, the items 1 to 3 mentioned below canbe also used as coloring materials for the color inks used in thepresent invention. These coloring materials are preferable because themajority of these coloring materials exhibit excellent water resistancewhen they are applied to a recording medium.

1. Dyes having a carboxyl group as a solubilizing group

2. Oil-soluble dyes

3. Pigments

Oil-soluble dyes are not particularly limited, as long as they aredescribed in the color index. Furthermore, it may be a novel dye that isnot described in the color index, not particularly limited. Specificexamples are described below. These dyes are used within the rangebetween 1% by mass and 10% by mass, and more preferably within the rangebetween 1% by mass and 5% by mass based on the total mass of the ink.

C. I. solvent blue: 33, 38, 42, 45, 53, 65, 67, 70, 104, 114, 115, 135

C. I. solvent red: 25, 31, 86, 92, 97, 118, 132, 160, 186, 187, 219

C. I. solvent yellow: 1, 49, 62, 74, 79, 82, 83, 89, 90, 120, 121, 151,153, 154

When a pigment is used as a coloring material for the color ink used inthe present invention, the pigment is used within the range between 1%by mass and 20% by mass, and more preferably within the range between 2%by mass and 12% by mass based on the total mass of the ink. Colororganic pigments that can be used in the present invention are asfollows.

Examples of a pigment used for a yellow ink may include C. I. PigmentYellow 1, C. I. Pigment Yellow 2, C. I. Pigment Yellow 3, C. I. PigmentYellow 13, C. I. Pigment Yellow 16, C. I. Pigment Yellow 74, C. I.Pigment Yellow 83, and C. I. Pigment Yellow 128.

Examples of a pigment used for a magenta ink may include C. I. PigmentRed 5, C. I. Pigment Red 7, C. I. Pigment Red 12, C. T. Pigment Red 48(Ca), C. I. Pigment Red 48 (Mn), C. I. Pigment Red 57 (Ca), C. I PigmentRed 112, and C. T. Pigment Red 122.

Examples of a pigment used for a cyan ink may include C. A. Pigment Blue1, C. I. Pigment Blue 2, C. I. Pigment Blue 3, C. I. Pigment Blue 15:3,C. I. Pigment Blue 16, C. I. Pigment Blue 22, C. I. Vat Blue 4, and C.T. Vat Blue 6.

However, pigments used in the present invention are not limited thereto.Other than the above listed pigments, naturally, a pigment newlyproduced for the present invention can also be used.

Moreover, when a pigment is used, any type of dispersant for dispersingthe pigment in the ink can be used together, as long as it is awater-soluble resin. A dispersant having a weight-average molecularweight within the range between 1,000 and 30,000 is preferable, and adispersant having a weight-average molecular weight within the rangebetween 3,000 and 15,000 is more preferable. Specific examples of such adispersant may include a block copolymer, a random copolymer, a graftcopolymer, and salts thereof, which consist of at least two monomers (atleast one of them being a hydrophilic monomer) selected from the groupconsisting of styrene and derivatives thereof, vinylnaphthalene andderivatives thereof, fatty acid alcohol esters of α,β-ethyleneunsaturated carboxylic acid, acrylic acid and derivatives thereof,maleic acid and derivatives thereof, itaconic acid and derivativesthereof, fumaric acid and derivatives thereof, and vinyl acetate,vinylpyrrolidone, acrylamide and their derivatives. Moreover, naturalresins such as rosin, shellac or starch are also preferably used. Theseresins are soluble in an aqueous solution in which bases are dissolved,and they are alkali-soluble resins. Such a water-soluble resin used as apigment dispersant is contained in the ink preferably within the rangebetween 0.1% by mass and 5% by mass based on the total mass of the ink.

An aqueous medium preferably used for the color ink used in the presentinvention is water, or a mixed solvent of water and a water-solubleorganic solvent. Such water is riot general water containing variousions, but it is preferably ion-exchanged water (deionized water).Examples of a water-soluble organic solvent used by mixing with watermay include: alkyl alcohols containing 1 to 4 carbon atoms such asmethyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol,n-butyl alcohol, sec-butyl alcohol or tert-butyl alcohol; amides such asdimethylformamide or dimethylacetamide; ketones or keto alcohols such asacetone or diacetone alcohol; ethers such as tetrahydrofuran or dixane;polyalkylene glycols such as polyethylene glycol or polypropyleneglycol; alkylene glycols whose alkylene group contains 2 to 6 carbonatoms, such as ethylene glycol, propylene glycol, butylene glycol,triethylene glycol, 1,2,6-hexanetriol, thiodiglycol, hexylene glycol ordiethylene glycol; glycerin; lower alkyl ethers of polyalcohols, such asethylene glycol monomethyl (or ethyl) ether, diethylene glycol methyl(or ethyl) ether, or triethylene glycol, monomethyl (or ethyl) ether;N-methyl-2-pyrrolidone, 2-pyrrolidone, and1,3-dimethyl-2-imidazolidinone. Among these many water-soluble organicsolvents, polyalcohols such as diethylene alcohol, and lower alkylethers of polyalcohols, such as triethylene glycol monomethyl (or ethyl)ether, are preferable.

The above-described water-soluble organic solvent is contained in thecolor ink generally within the range between 3% by mass and 50% by mass,and preferably within the range between 3% by mass and 40% by mass basedon the total mass of the ink. Moreover, water used herein is containedin the ink within the range between 10% by mass and 90% by mass, andpreferably within the range between 30% by mass and 80% by mass based onthe total mass of the ink. Furthermore, in order that the color ink usedin the present inventIon has desired physical property values asnecessary, it may comprise, as appropriate, various additives such as asurfactant, anti-foaming agent or antiseptic agent, as well as the abovecomponents

The black and color inks used in the present invention containing theabove-described components preferably have good dischargeability from anink jet recording head. Accordingly, from the viewpoint ofdischargeability from an ink jet recording head, the above inkspreferably have properties such as a viscosity of 1 to 15 mPa/s and asurface tension of 25 mN/m or more, and more preferably properties suchas a viscosity of 1 to 5 mPa/s and a surface tension of 25 to 50 mN/m.In the case of the combined use of a black ink and a color ink, thesurface tension of the color ink is preferably lower than that of theblack ink. More specifically, the surface tension if the black ink isbetween 35 and 50 mN/m, and the surface tension of the color ink isbetween 25 and 35 mN/m.

<Image Forming Method>

The image forming method of the present invention will be explained withthe following specific examples. The image forming method of the presentinvention is characterized in that it uses the water-based ink of thepresent invention having the above structure as a black ink, and inorder to form an image in which an image formed with the black ink andan image formed with a color ink are adjacent, first scanning forattaching the black ink is carried out, and then scanning for attachingthe color ink to the region in which the black image has already beenformed. The specific means will be explained below.

FIG. 8 shows an example of a recording head that is used for the imageforming method of the present invention. As shown in FIG. 8, therecording head has a line of discharge orifices for ejecting a black ink(Bk) and lines of discharge orifices for ejecting each of three colorinks, cyan (C), magenta (M) and yellow (Y). When a color image is formedby the image forming method of the present invention, it is preferableto use a recording head in which the orifice line for ejecting the blackink and the orifice lines for ejecting the color inks do not overlapalong the sub-scanning (paper-feed) direction. Accordingly, in order toform an image using the recording head shown in FIG. 8 for example, whenonly a black image is formed, the whole area of the black orifice lineis used, and when a color image is formed with both black and colorinks, it is preferable to use the portion a of the black orifice lineshown in FIG. 8 and the portions b of the orifice lines for C, M and Y.The case of forming an image consisting of both black ink and color inkswill be explained below in more detail, using FIG. 8.

In FIG. 8, first, using the portion a of the black ink discharge orificeline, the print head is scanned in the horizontal direction (mainscanning direction), so that a black image is formed on a recordingmedium such as a plain paper by one-pass printing. Subsequently, therecording medium is moved in the vertical direction (sub-scanningdirection) shown in FIG. 8 by the distance a. In the next step, usingthe portion b of each color ink orifice line, a color image is formed byone-pass printing in the forward scanning direction onto a region inwhich the black image has been formed using the orifices of portion a.At this time, the black ink discharge orifices in the portion asimultaneously form another black image onto the next region. Repeatingthis operation, an image consisting of both black ink and color inks isformed.

FIG. 9 shows another example of a recording head that is used for theimage forming method of the present invention. As in the case of FIG. 8,in FIG. 9 also, the black ink orifices in the portion a are used to forma black image, and the portion b corresponding to the whole area of eachcolor ink discharge orifice line is used for C, M and Y, and thus, animage consisting of both black ink and color inks is formed in the samemanner as described for FIG. 8.

FIG. 10 shows another example of a recording head that can be used inthe image forming method of the present invention. As in the case ofFIG. 8, in FIG. 10 also, the portion a of the black ink dischargeorifices is used for black image formation, and the portion bcorresponding to all orifices for each color ink of C, M and Y is usedfor color image formation, and thus, an image consisting of both blackink and color inks is formed. In the recording head shown in FIG. 10,the portion a of the black ink discharge orifice line and the portion bof the color ink discharge orifice lines are apart by a distancecorresponding to the distance a of single paper conveyance as shown inFIG. 10. Accordingly, the recording head with such a structure generatesa time .difference, which corresponds to a time necessary for a singlereciprocating print scanning, from when a black image is formed untilwhen a color image is formed. Accordingly, it can be said that therecording head shown in. FIG. 10 has a structure more advantageous thanthat of the recording head shown in FIG. 9 in terms of bleeding betweenblack ink and color inks.

FIG. 11 shows another example of a recording head that is used in theimage forming method of the present invention. As shown in the figure,this recording head is configured such that discharge orifice lines forboth black ink and color inks are aligned in a line in a paper feedingdirection. On such a case also, a color image is formed after a blackimage is formed, corresponding to paper feeding.

FIG. 12 shows another example of a recording head that is used in theimage forming method of the present invention. The recording head shownin FIG. 12 is configured such that for each color ink two dischargeorifice lines are provided, i.e., cyan (C1, C2), magenta (M1, M2) andyellow (Y1, Y2), in a symmetric arrangement in the main scanningdirection, so that the landing order of the color inks becomes the sameboth in the forward scanning and in the backward scanning. As a result,bidirectional printing becomes possible even when an image consisting ofboth the black ink and the color inks is formed. In this case, using theportion a of the black ink, a black image is first formed in the forwarddirection of the main scanning direction of the print head. Thereafter,a recording medium is carried by a distance a. Thereafter, using theportion b of color ink discharge orifice lines, the head forms a colorimage by one pass printing during the backward scanning In the mainscanning direction on a region where the black image has been formed bythe orifices in the port ion a. At this time, the portion a of the blackink orifices forms another black image onto the next region. Repeatingthis operation, an image consisting of both black ink and color inks isformed.

The head for bidirectional printing shown in FIG. 12 may also beconfigured such that both the black nozzles and the color nozzles arearranged to provide an interval of single scanning between the blackimage formation and the color image formation as described above, so asto be more advantageous in anti-bleeding effect (refer to FIG. 13) Theimage forming method of the present invention is described above, butthe form of a recording head used for the method of the presentinvention is not limited to those shown in FIGS. 8 to 13.

[Recording Mmethod, Recording Unit, Cartridge, and Recording Apparatus]

Next, an example of an ink jet recording apparatus preferably used inthe present invention will be described. First, an example of thestructure of a head that is a principal part of an ink jet recordingapparatus using thermal energy is shown in FIGS. 1 and 2. FIG. 1 is asectional view of a head 13 shown along the ink channel, and FIG. 2 is asectional view along the A-B line of FIG. 1. The head 13 is formed byattaching glass, ceramic, silicon, or plastic etc. in which at lease oneink flow path 14 is provided, to a heat generating element 15 (notlimited to what is shown in Figures). The heat-generating element 15 iscomposed of a protective film 16 made of silicon oxide, siliconenitride, silicon carbide or the like, aluminum electrodes 17-1 and 17-2made of aluminum, aurum, aluminum-copper alloy or the like, aheat-generating resistance layer 18 made of HfB₂, TaN, TaAl etc., aheat-accumulating layer 19 made of thermal oxidized silicon, oxidizedaluminum etc., and a substrate 20 made of silicon, aluminum, aluminumnitride etc. which is excellent in heat releasing Upon application ofthe electric signal to the electrodes 17-1 and 17-2 as a pulse, heat israpidly generated at the region shown by “n” to form a bubble in the ink21 in contact with this region. The meniscus 23 of the ink is ejected bythe pressure of the bubble thus produced, and is ejected from theorifice 22 to a recording medium (for example, paper) 25 in the form ofan ink droplet 24 to attach onto the recording medium 25. FIG. 3schematically illustrates a recording head having an array of nozzlessimilar to that is shown in FIG. 1. This head is prepared by bonding aglass plate 27 having a number of flow path 26 to a heat generating head28 similar to that is shown in FIG. 1.

FIG. 4 illustrates an example of an ink-jet recording apparatus in whichsuch a head as described above is incorporated. In FIG. 4, the blade 61is a wiping member, one end of which is a fixed end held by ablade-holding member to cantilever. The blade 61 is provided at aposition adjacent to a region in which a recording head 65 operates, andin this aspect, is held in such a form that it protrudes into the pathof the recording head 65. Reference numeral 62 designates a cap for anejection opening of the recording head 65, and the cap is arranged in ahome position adjacent to the blade 61, moves in the directionperpendicular to the moving direction of the recording head 65, and capsthe ink-ejecting opening when touching it. Numeral 63 designates anink-absorber provided adjacent to the blade 61, which is held in themoving path of the recording head 65 in a projecting form like the blade61. The above blade 61, cap 62 and ink-absorber 63 constitute anejection recovery part 64, and the blade 61 and the ink-absorber 63serve to remove moisture and dust on the ink-ejecting opening.

Reference numeral 65 designates a recording head. The head contains anenergy generating means for ink ejection and performs recording byejecting ink towards a recording medium opposite to the ink-ejectingopening. Numeral 66 designates a carriage for carrying the recordinghead 65 to move it. The carriage 66 is engaged with a guide shaft 67 ina slidable manner and a part of the carriage 66 is connected to a belt69 (not shown in the figure) driven by a motor 68. Thus the carriage 66can move along the guide shaft 67, and the recording head 65 can move inthe recording region and the region adjacent thereto.

Reference numeral 51 designates a recording medium feeding part forinserting a recording medium and numeral 52 designates a paper-deliveryroller driven by a motor not shown in the figure. With such anarrangement, the recording medium is fed to the position opposite to theink ejecting opening of the recording head 65 and conveyed to a paperoutput portion provided with a paper output roller 53 as recordingproceeds. In the image recording apparatus according to this aspect ofthe invention, the recording head is moved forward and backward along adirection perpendicular to the recording medium-carrying direction, andin the both forward and backward ways, the head can apply at least oneof black ink and color ink to the recording medium. Recording dataprocessing may be done utilizing conventional technology relating to theboth-way printing.

In the above arrangement, while the recording head returns to its homeposition after recording, the cap 62 of the ejection recovery part 64recedes from the moving path of the recording head, but the blade 61 isprojecting in the moving path. As a results the ink discharge opening ofthe recording head 65 is wiped. As a result, the discharge orifice faceof the recording head 65 is wiped also during this movement. The abovemovement of the recording head 65 to its home position is made not onlywhen the recording is completed or for ejection recovery, but also whenthe recording head 65 is moving in the recording region for recording,that is, it moves to the home position adjacent to the recording regionat given intervals during recording, so as to wipe the ejection openingface with this movement.

FIG. 5 shows one example of an ink cartridge for storing ink for feedingink to the recording head through an ink feeding member such as a tube.In the drawing, reference numeral 40 denotes a member Constituting theink cartridge 45, an ink storage portion such as an ink bag, whose tipis equipped with a rubber stopper 42. The ink in the ink bag 40 can befed to the recording head by inserting a needle (not shown in thefigure) into the stopper 42. Numeral 44 designate an ink absorber forreceiving waste ink. For the ink storage portion, its surface in contactwith ink is preferably made of polyolefin, particularly polyethylene.

The ink-jet recording apparatus used in the present invention are notlimited to the apparatus as described above in which the head and theink cartridge are separately provided. Therefore, a device in whichthese members are integrally formed as shown in FIG. 6 can also bepreferably used. In FIG. 6, reference numeral 70 designates a recordingunit including an ink storing portion containing an ink, for example, anink-absorbing member in it. The ink contained in the ink-absorbingmember is ejected as an ink droplet from a head 71 having a plurality oforifices. As a material for the ink-absorbing member, polyurethane maybe preferably used. Reference numeral 72 indicates an air passage forcommunicating the interior of the recording unit 70 with the atmosphere.This recording unit 70 can be used in place of the recording head 65shown in FIG. 4, and detachably installed on the carriage 66.

A configuration example of the recording head utilizing mechanicalenergy is an on-demand ink jet recording head comprising: anozzle-forming substrate having a plurality of nozzles, apressure-generating element consisting of a piezoelectric material andarm electric conductive material opposed to the nozzles and an inkfilling in the peripheral portion of the pressure-generating element, inwhich the pressure-generating element is displaced by applying voltage,so as to eject ink droplets from the nozzle.

In FIG. 7, the ink flow path 80 is made from a photosensitive resin; theorifice plate 81 which is made of a metal such as stainless steel andnickel; the ejection orifice 85 which is formed by making a hole in theorifice plate 81 by electrocasting or press processing; the vibrationplate 82 which is made from a metal film such as stainless steel, nickeland titanium and a highly elastic resin film; and the piezoelectricelement 83 which is made from a dielectric material such as bariumtitanate and PZT. The recording head of the above constitution works asfollows: when a pulse-like voltage is given to the piezoelectric element83, a strain stress is generated, of which energy deforms the vibrationplate connected with the piezoelectric element 83 to apply pressurevertically to the ink in the ink flow path 80, and an ink droplet (notshown) is discharged from the orifice 85 of the orifice plate 81 toperform recording. Such a recording head is incorporated into thenrecording apparatus similar to the one shown in FIG. 4. Detailed actionof the parts of the recording apparatus may be the same as mentionedabove. FIG. 7 shows, air example of the structure of the recording headthat is the principal part of the recording apparatus.

The head is comprised of an ink channel 80 connecting to an ink chamber(not shown in the figure), an orifice plate 81 for ejecting a desiredvolume of ink droplets, a diaphragm 82 for directly applying pressure tothe ink, a piezoelectric element 83 that is connected to the diaphragm82 and is displaced by an electronic signal, and a substrate 84 forsupporting and fixing the orifice plate 81, the diaphragm 82 or thelike.

In FIG. 71 the ink channel 80 is made of a photosensitive resin or thelike, and a discharge port 85 is formed through the orifice plate 81made of metal such as stainless or nickel, by drilling or the like, suchas electroforming or press work. The diaphragm 82 is made of a metalfilm such as stainless, nickel or titanium and a highly elastic resinfilm. The piezoelectric element 83 is made of a dielectric material suchas barium titanate or PZT. The recording head with the above structuregives a pulsed voltage to the piezoelectric element 83 to allow it togenerate distortion stress. Then, the generated energy deforms thediaphragm connected to the piezoelectric element 83, thereby verticallypressurizing the ink contained in the ink channel 80, so that inkdroplets (not shown in the figure) are elected from the dischargeorifice 85 on the orifice plate 81, so as to perform printing Thisrecording head is incorporated into an ink jet recording apparatussimilar to that shown in FIG. 4, and used. The detailed actions of theink jet recording apparatus are the same as described above.

EXAMPLES

The present invention will be described more specifically with referenceto the following examples and comparative examples. However, the presentinvention is not limited by the following examples, as lone as it is inthe scope of the invention. It is to be noted that “part” and “%” arebased on mass criteria in the present specification, unless otherwisespecified.

(Preparation of Pigment Dispersion Solution 1)

Ten parts of carbon black having a specific surface area of 210 m²/g anda DBP oil absorption of 74 ml/100 g, 20 parts of a 10% styrene-acrylicacid copolymer (acid value: 200, weight-average molecular weight:10,000) in water neutralized with sodium hydroxide, and 70 parts of ionexchanged water were mixed. The mixture was dispersed for 1 hour using asand grinder. Thereafter, coarse particles were removed by centrifugalseparation, and the supernatant was subjected to pressure filtrationusing a microfilter with a pore size of 3.0 μm (manufactured by FujiPhoto Film Co., Ltd.) to obtain a pigment dispersion solution 1dispersed with a resin. The obtained pigment dispersion solution 1 had asolid content of 10, a pH of 10.0 and an average particle size of 120nm.

(Preparation of Pigment Dispersion Solution 2)

Carbon black (10 g) having a specific surface area of 230 m²/g and a DBPoil absorption of 70 ml/100 g, 3.41 g of p-amino-N-benzoic acid, and 72g of water were fully mixed, to which 1.62 g of nitric acid was addeddropwise and stirred at 70° C. Minutes later, a solution of 1.07 gsodium nitrite in 5 g of water was added to the mixture, followed byfurther stirring for 1 hour. The obtained slurry was filtrated with Toyofilter No. 2 (manufactured by Advantis), and pigment particles werefully washed with water and then dried at 90° C. in an oven. Thereafter,water was added to this pigment to prepare a pigment aqueous solution of10% pigment concentration. The thus obtained pigment dispersion solution2 contained anionically charged self-dispersing carbon black having ahydrophilic group bonded to the surfaces of the pigment particles via aphenyl group.

The density of ionic groups of the thus prepared self-dispersing carbonblack measured by the following method was 1.3 μmol/m². Theconcentration of sodium ions was measured using an ion meter(manufactured by DKK), and the obtained value was converted into thedensity or ionic groups. By the above-described method, a pigmentdispersion solution 2 was obtained, in which self-dispersing carbon wasobtained by introducing a —C₆H₄—COONa group into the surface of carbonblack.

(Preparation of Pigment Dispersion Solution 3)

Carbon black (500 g) having a specific surface area of 220 m2/g and aDBP oil absorption of 112 ml/100 g, 45 g ofaminophenyl(2-sulfoethyl)sulfone (APSES), and 900 g of distilled waterwere placed in a reactor, and while keeping the temperature at 55° C.,the mixture was stirred at 300 RPM for 20 minutes. Thereafter, 40 g of25% sodium nitrite was added dropwise to the mixture for 15 minutes, andthen 50 g of distilled water was added thereto. Thereafter, keeping thetemperature at 60° C., a reaction was carried out for 2 hours.Thereafter, the reaction product was taken out while diluting withdistilled water to a solid content of 15%. Thereafter, centrifugalseparation and a purification treatment to remove impurities werecarried out. In the thus prepared dispersion solution, the functionalgroup of the above APSES was bonded to the carbon black. This dispersionsolution was called Al.

Subsequently, in order to determine the mole number of the functionalgroup bonded to the carbon black in the dispersion solution Al, Na ionsin the dispersion solution were measured with a probe-type sodiumelectrode. The obtained value was converted per carbon black particle,so as to obtain the mole number of the functional group bonded to thecarbon black. Thereafter, the previously prepared dispersion solution Alwith a solid content of 15% was added dropwise into apentaethylenehexamine (PEHA) solution. During this process, the PEHAsolution was intensively stirred while keeping at room temperature, andthe dispersion solution Al was added dropwise thereto over 1 hour.During this process, the concentration of PEHA was set to 1 to 10 timesof the previously determined mole number of Na ions, and the amount ofthe PEHA solution was set to be equivalent to the amount of thedispersion solution Al. Thereafter, the obtained mixture was stirred for18 to 48 hours, and then a purification treatment was carried out toeliminate impurities. Finally, a dispersion of carbon black to whichpentaethylenehexamine (PEHA) was obtained at a solid content of 10%.This dispersion solution was called B1.

Subsequently, a copolymer styrene-acrylic acid resin solution wasprepared by weighing 190 g of styrene-acrylic acid resin having aweight-average molecular weight of 8,000, an acid value of 140, and apolydispersity Mw/Mn (weight-average molecular weight Mw/number-averagemolecular weight Mn) of 1.5, to which 1,800 g of distilled water andNaOH necessary to neutralize the resin were added and stirred.Thereafter, while stirring, 500 g of the previously prepared dispersionsolution B1 of a solid content of 10% was added dropwise to the abovestyrene-acrylic acid resin aqueous solution. Thereafter the mixture ofB1 and the styrene-acrylic acid resin aqueous solution was transferredto an evaporating dish of Pyrex (trademark), and it was then heated at150° C. for 15 hours for evaporation. The dried matter obtained afterthe evaporation was cooled to room temperature.

Subsequently, using a dispersing device, the dried matter obtained afterthe evaporation was dispersed in distilled water adjusted to pH 9.0 withNaOH. While further stirring, 1.0 M NaOH was added to the solution toadjust the pH of the solution to 10-11. Pigment dispersion solution 3was obtained after the desalting and purification of the above solutionand removal or coarse particles. The pigment dispersion solution 3 had asolid content of 10%, a pH of 10.1 and an average particle size of 130nm. The following is a scheme of synthesizing a. polymer-bindingself-dispersing pigment in which an organic group is chemically bondedto the surface of a carbon black particle contained in the above pigmentdispersion solution 3.

Scheme of Synthesizing Modified Pigment

[Method of Determining Whether the Used Water-Soluble Organic Solvent isGood Solvent or Poor Solvent]

In order to determine whether the used water-soluble organic solvent isa good solvent or poor solvent to a pigment, or a pigment and adispersant contained, in the above pigment dispersion solution, thefollowing experiment was carried out. First, with each of the abovepigment dispersion solutions 1 to 3 of 10% solid content, a testdispersion solution for determining good or poor solvent was prepared atthe following formulation.

[Mixing Ratio for Dispersion Solution for Determining Good or PoorSolvent] Each pigment dispersion solution of 10% solid 50 parts content:Each water-soluble organic solvent described 50 parts in Table 1:

(Determination)

10 g of the above-prepared test solution for determining good or poororganic solvent was placed in a sample jar equipped with a transparentglass lid. After the jar was covered with the glass lid, the solutionwas stirred well, and left standing in an oven at 60° C. for 48 hoursThereafter, the dispersion solution was taken out of the oven to be usedas a measurement sample. The particle size of the water-insolublecoloring material contained in the solution was measured using a richsolution particle size analyzer (Product name: FPAR-1000; manufacturedby Otsuka Electronics) The obtained value denoted the particle size ofan undiluted solution (the particle size measured without diluting thesolution) of the dispersion solution used to determine whether theorganic solvent is a good solvent or poor solvent after storage at 60°C. for 48 hours. Separately, reference pigment dispersions were preparedin the same manner as the preparation of the test dispersion solutionsexcept that the water-soluble organic solvent was replaced with the sameamount of water. The particle size of the water-insoluble coloringmaterial in the undiluted reference solution was easured by using therich solution particle size analyzer hut without heating-storage of thesolution. The obtained particle size of the undiluted test solution wascompared with that of the reference dispersion solution. When the formerparticle size was larger than the latter, the contained solvent wasdetermined to be a poor solvent. In contrast, when the former particlesize was equal to or smaller than the latter, it was determined to be agood solvent.

[Method of Measuring Ka Value of Each Water-Soluble Organic Solvent]

First, in order to facilitate the measurement of the Ka value of eachwater-soluble organic solvent, a 0.5% dye solution in water of thefollowing composition was prepared. Water-soluble dye C. I. direct blue199  0.5 parts Pure water 99.5 parts

Subsequently, using the above 0.5% dye solution, colored aqueoussolutions respectively containing the water-soluble organic solvents tobe measured at 20% were prepared with the following formulation: Theabove 0.5% dye aqueous solution 80 parts Each water-soluble organicsolvent described 20 parts. in Table 1

Using the above-prepared colored aqueous solution containing 20% ofwater-soluble organic solvent, the Ka value was determined by theBristow method, by using Dynamic Permeability Tester S (product name)manufactured by Toyo Seiki Seisaku-sho, Ltd.

<<Determination and Determination Results>>

Table 1 shows the results obtained by determining whether or not each ofthe above measured water-soluble organic solvents usable for the ink isa good solvent or poor solvent to each of the pigment dispersionsolutions 1 to 3, and the measurement results of the Ka value of each20% water-soluble organic solvent-containing aqueous solution. In Table1, the polyethylene glycol derivative denotes a derivative of thefollowing structure, having a molecular weight of approximately 1,000:

wherein each of n and m represents a number between 5 and 20. TABLE 1Determination results whether or not each water-soluble organic solventis good solvent or poor solvent, and Ka values Water- Ka value solubleWater-insoluble coloring material of 20% organic Pigment Pigment Pigmentaqueous solvent dispersion 1 dispersion 2 dispersion 3 solution Glycerin◯ ◯ ◯ 0.13 Ethylene ◯ ◯ ◯ 0.09 glycol Diethylene X X X 0.14 glycolTrimethylol ◯ ◯ ◯ 0.19 propane Polyethylene X X X 0.17 glycol 600Polyethylene X X X 0.18 glycol derivative 2- X ◯ X 0.19 pyrrolidoneIn the table, ◯: good solvent, X: poor solvent

EXAMPLES 1 TO 5

The above-described water-soluble organic solvents, pigment dispersionsolutions 1 to 3, and components described in Table 2 were mixed andfully stirred for dissolution or dispersion. Thereafter, the obtainedmixture was subjected to pressure filtration using a microfilter with apore size of 3.0 μm (manufactured by Fuji Photo Film Co., Ltd.), so asto prepare inks of Examples 1 to 5. The inks were prepared such thatwhen the total amount (% by mass) of the good solvent in the ink wasrepresented by A and the total amount (% by mass) of the poor solvent inthe ink was represented by B, the ratio of A to B was within the rangebetween 10:5 or more and 10:30 or less, and when Ka values of theplurality of water-soluble organic solvents were determined by theBristow method and the obtained values were compared with each other, awater-soluble organic solvent having the largest Ka value was a poorsolvent. TABLE 2 Composition of inks of Examples 1 to 5 Composition (%by mass) Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Water insoluble Pigmentdispersion 1 50 — — — — coloring material Pigment dispersion 2 — 50 — 50— Pigment dispersion 3 — — 50 — 50 Water Good Glycerin 5 5 5 7 5 solublesolvent Ethylene glycol 5 — — — — organic Trimethylol propane — — — — —solvent Poor Diethylene glycol — — 5 3 3 solvent Polyethylene glycol 60010 15 10 6 — Polyethylene glycol derivative — — — — 10 SurfactantAcetylene glycol EO adduct(*) 0.1 0.1 0.1 0.1 0.1 Pure water 29.9 29.929.9 33.9 31.9(*)Product name: Acetylenol EH manufactured by Kawaken Fine ChemicalsCo., Ltd.

<Verification of Ink of Example 4>

Assuming that the composition of the above-prepared ink of Example 4 wasunknown, whether or not the ink was the object of the present inventionwas verified by the following method. By this verification method, evenif the composition of the ink is unknown, it can be easily confirmedwhether or not the ink is the object of the present invention.

The type and amount of organic solvents contained in the ink can beidentified using, for example, GC/MS (product name: TRACE DSQ,manufactured by ThermoQuest). More specifically, for example, a sampleobtained by diluting 1 g of the ink of Example 4 with methanol wasanalyzed using the above CC/MS. As a result, first of all,, theexistence of glycerin, diethylene glycol and polyethylene glycol 600 wasconfirmed in the sample. Then, it was necessary to determine whetherthese three types of solvents were good solvents or poor solvents. Inthe above-described method of determining whether a solvent is a goodsolvent or poor solvent, a dispersion solution in which awater-insoluble coloring material was dispersed to the solvent and waterwas prepared. However, in order to prepare such a dispersion solutionfrom the ink, it is necessary to extract a water-insoluble coloringmaterial or dispersant from the ink. However, in this case, thewater-insoluble coloring material or dispersant might be deterioratedduring the extraction process.

Thus, the present inventors have made various studies to find a method,which involves the direct use of the ink of Example 4 to determinewhether the solvent contained therein is a good solvent or poor solventand provides determination results that are consistent with the resultsof the above-described determination method. As a result, the presentinventors have found that the following verification method ispreferable. First, the following three types of ink dilutions wereprepared by adding each of the above three types of water-solubleorganic solvents to be determined in an equal amount to 100 parts of theink of Example 4, and determination was carried out using thesesolutions. That is to say, 3 types of ink dilutions of compositionsshown in Table 3 (shown as Verification examples 1 to 3 in Table 3) wereprepared, each of which contained approximately 50% by mass ofwater-soluble organic solvent as a determination target. Thereafter,these solutions were kept standing at 60° C. for 48 hours, and theparticle sizes of water-insoluble coloring materials contained thereinwere then measured using a rich solution particle size analyzer (Productname: FPAR-1000; manufactured by Otsuka Electronics Co Ltd.). On theother hand, the ink that was not subjected to the storage under heatingwas also measured in terms of the particle size of the water-insolublecoloring material contained therein. Determination of a good solvent orpoor solvent was carried out in accordance with the criteria that themeasurement value of the particle size after the storage under heatingis compared with each other with that of the ink without storage, andthat when the former particle size is greater than the latter particlesize, it is determined to be a poor solvent, and when the formerparticle size is equal to or smaller than the latter particle size, itis determined to be a good solvent. In Table 3 shown below, theviscosity (cP) of the used solvents is also shown as a measurementcondition for the particle size in an undiluted solution. The viscositywas measured with a type E viscometer (VISCONIC type ED, manufactured byTokyo Keiki Co., Ltd.) TABLE 3 Composition of test samples fordetermining good solvent or poor solvent regarding solvents used for inkof Example 4 Composition of test Ink of Examples [parts] Example 4 1 2 3Ink of Example 4 — 100 100 100 Glycerin — 100 — — Diethylene glycol — —100 — Polyethylene — — — 100 glycol 600 Viscosity [cP] 2.3 10.1 8.7 33.9

Table 4 shows the pigment particle size in the undiluted ink of Example4 without storage and the pigment-particle size of each undilutedsolution of Verification examples 1 to 3 after storage under heating at60° C. for 48 hours, which were measured as described above. Comparingthe measurement values, when the measurement value of the particle sizein each of the verification examples was larger than that of the ink ofexample 4, it was determined to be a poor solvent, and when themeasurement value of the particle size in each of the verificationexamples was equal to or smaller than that of the ink of Example 4, itwas determined to be a Blood solvents TABLE 4 Test results of solventscontained in ink of Example 4 Particle size in undiluted solutionMeasurement sample [nm] Determination Test example 1 71.2 ◯ (afterstorage under heating) Test example 2 167.8 X (after storage underheating) Test example 3 160.7 X (after storage under heating) Ink ofExample 4 71.9 — (without storage)Conditions for storage under heating: 60° C., 48 hours◯: good solvent, X: poor solvent.

As shown in Table 4, the results of the method of determining a goodsolvent or poor solvent using verification samples obtained by dilutinga prepared ink can confirm that glycerin (used in Verificationexample 1) was a good solvent, and diethylene glycol (used inVerification example 2) and polyethylene glycol 600 (used inVerification example 3) were poor solvents, as with the determinationmethod shown in Table 1, that is, the same results were obtained by theabove two different types of determination methods. Accordingly, it wasconfirmed that the method of using an actual ink to determine whetherthe solvent used therein is a good solvent or poor solvent to thecoloring material contained therein is effective. Therefore, the abovemethod for determining whether the used solvent is a good solvent orpoor solvent using a sample obtained by diluting the ink can alsoeffectively be used in the present invention.

COMPARATIVE EXAMPLES 1 TO 17

(Preparation of Inks)

The above-described water-soluble organic solvents, pigment dispersionsolutions 1 to 3, and components shown in Tables 5-1 to 5-3 were mixed.The obtained mixture was fully stirred for dissolution or dispersion,and it was then subjected to pressure filtration using a microfilterwith a pore size of 3.0 μm (manufactured by Fuji Photo Film Co., Ltd.),so as to obtain inks of Comparative Examples 1 to 17. TABLE 5-1Composition of inks of Comparative Examples 1 to 6 Composition of inksof Comparative Examples (% by mass) 1 2 3 4 5 6 Water insoluble Pigmentdispersion 1 50 50 50 — — — coloring material Pigment dispersion 2 — — —50 50 50 Pigment dispersion 3 — — — — — — Water Good Glycerin 5 — 5 5 —5 soluble solvent Ethylene glycol — — 5 — — 5 organic Trimethylolpropane 7 — 5 7 — 5 solvent Poor Diethylene glycol — — — — — — solventPolyethylene glycol 600 10 15 — 10 15 — Polyethylene glycol derivative —— — — — — Surfactant Acetylene glycol EO adduct(*) 0.1 0.1 0.1 0.1 0.10.1 Pure water 27.9 34.9 34.9 27.9 34.9 34.9(*)Product name: Acetylenol EH manufactured by Kawaken Fine ChemicalsCo., Ltd.

TABLE 5-2 Composition of inks of Comparative Examples 7 to 12Composition of inks of Comparative Examples (% by mass) 7 8 9 10 11 12Water-insoluble Pigment dispersion 1 — — — — 50 — coloring materialPigment dispersion 2 — — — — — 50 Pigment dispersion 3 50 50 50 50 — —Water Good Glycerin 5 — 5 — 3 3 soluble solvent Ethylene glycol — — 5 —— — organic Trimethylol propane 7 — 5 — — — solvent Poor Diethyleneglycol — — — — — — solvent Polyethylene glycol 600 10 15 — — — —Polyethylene glycol derivative — — — — 15 15 Surfactant Acetylene glycolEO adduct(*) 0.1 0.1 0.1 0.1 0.1 0.1 Pure water 27.9 34.9 34.9 39.9 31.931.9(*)Product name: Acetylenol EH manufactured by Kawaken Fine ChemicalsCo., Ltd.

TABLE 5-3 Composition of inks of Comparative Examples 13 to 17Composition of inks of Comparative Examples (% by mass) 13 14 15 16 17Water-insoluble Pigment dispersion 1 — 50 — — — coloring materialPigment dispersion 2 — — 50 — 50 Pigment dispersion 3 50 — — 50 — WaterGood Glycerin 3 5 5 5 15 soluble solvent Ethylene glycol — 8 8 8 —organic Trimethylol propane — — — — — solvent Poor Diethylene glycol — —— — — solvent Polyethylene glycol 600 — 5 5 5 — Polyethylene glycolderivative 15 — — — — Surfactant Acetylene glycol EO adduct(*) 0.1 0.10.1 0.1 0.1 Pure water 31.9 31.9 31.9 31.9 34.9(*)Product name: Acetylenol EH manufactured by Kawaken Fine ChemicalsCo., Ltd.

<Evaluation of Image Properties>

The inks of Examples 1 to 5 and Comparative Examples 1 to 17 wereevaluated in terms of properties described below, using a modified inkjet recording apparatus BJS-700 (manufactured by Canon Inc.) having anon-demand multi-recording head, which applies thermal energy to ink inresponse to a recording signal so as to eject the ink. The obtainedevaluation results regarding examples are shown in Table 6, and thoseregarding Comparative Examples are shown in Table 7.

1. Print density

Using the above inks and the above ink jet recording apparatus,characters including a solid area of 2 cm×2 cm were printed on plaincopy papers A to E described below. On the day following the printing,the print density of the solid area of 2 cm×2 cm was measured. Theprinter driver mode was a default mode. The setting of the default modewas shown below. The amount of ink ejected per ink dot was within 30ng±10%.

Type of paper: plain paper

Printing quality: standard

Color adjustment: automatic

The print density obtained as a result of the above measurement wasevaluated in accordance with the following criteria:

◯: The average print density of the 5 papers was 1.4 or higher, and theprint density of the paper with the lowest print density was 1.25 orhigher.

Δ: The average print density of the 5 papers was 1.4 or higher, and theprint density of the paper with the lowest print density was less than1.25.

X: The average print density of the 5 papers was less than 1.4.

The following copy papers were used in the above printing test:

A: PPC paper NSK, manufactured by Canon Inc.,

B: PPC paper NDK, manufactured by Canon Inc.,

C: C: PPC paper 4024, manufactured by Fuji Xerox Co., Ltd.,

D: PPC paper Prober Bond, manufactured by Fox River Paper Co., and

E: Canon PPC paper, manufactured by Die Neusiedler AG.

2. Print Density When Printed With a Small Amount of Ink

Print density with less ink was measured in the same manner as aboveexcept that the ejection amount of ink per dot was set within the rangeof 24 ng±10%. Thereafter, the obtained print density was evaluatedaccording to the same criteria as above.

<Evaluation of Storage Stability>

Each of the ink of Examples 1 to 5 and

Comparative Examples 1 to 17 was placed into a shot bottle and thebottle was hermetically closed The bottle was then placed in an oven at60° C. Two weeks later, the bottle was taken out and the state of theink was then observed. The storage stability was evaluated in accordancewith the criteria described below. The obtained evaluation results withinks of Examples are shown in Table 6, and those with inks ofComparative Examples are shown in Table 7.

◯: Color materials contained in the ink are dispersed stably anduniformly.

X: The ink has become gel, or the upper portion thereof becomestransparent, or it is clearly thickened. TABLE 6 Evaluation of ExamplesPrint density Normal ejection Small ejection Storage of ink of inkstability Example 1 ◯ ◯ ◯ Example 2 ◯ ◯ ◯ Example 3 ◯ ◯ ◯ Example 4 ◯ ◯◯ Example 5 ◯ ◯ ◯

TABLE 7 Evaluation results of Comparative Examples Print density Normalejection Small ejection Storage of ink of ink stability Comparative X X◯ Example 1 Comparative ◯ Δ X Example 2 Comparative X X ◯ Example 3Comparative X X ◯ Example 4 Comparative ◯ Δ ◯ Example 5 Comparative X X◯ Example 6 Comparative X X ◯ Example 7 Comparative ◯ Δ X Example 8Comparative X X ◯ Example 9 Comparative ◯ Δ X Example 10 Comparative ◯ ΔX Example 11 Comparative ◯ Δ ◯ Example 12 Comparative ◯ Δ X Example 13Comparative X X ◯ Example 14 Comparative X X ◯ Example 15 Comparative XX ◯ Example 16 Comparative X X ◯ Example 17

<Measurement of Ink Landing on Plain Paper

(Preparation of Inks of Example and Comparative Examples for Measurementof Ink Landing)

The pigment dispersion solution 2 was used, and it was mixed withcomponents shown in Table 8. The mixture was fully stirred fordissolution or dispersion, and it was subjected to pressure filtrationusing a microfilter with a pore size of 3.0 μm (manufactured by FujiPhoto Film Co., Ltd.), so as to obtain each of the inks of Example 6 andComparative Examples 18 to 20. In preparation of these inks for inklanding measurement, a water-soluble dye C. I. direct blue 199 was usedto visually observe the spread of the ink after it fixed on a recordingmedium. The surface tension of each of these inks is also shown in Table8. The surface tension was measured with a tension meter CBVP-A3manufactured by Kyowa Interface Science Co., Ltd., at a measurementtemperature of 25.0±0.5° C., using a platinum plate of 10 mm×24 mm.TABLE 8 Composition of inks of Example and Comparative Examples formeasuring ink landing Composition (% by mass) Com. Ex. Com. Ex. Com. Ex.Ex. 6 18 19 20 Water-insoluble Pigment dispersion 1 — — — — coloringmaterial Pigment dispersion 2 50 50 50 50 Pigment dispersion 3 — — — —Water Good Glycerin 7.5 — 15 — soluble solvent Ethylene glycol — — — —organic Trimethylol propane — — — 7.5 solvent Poor Diethylene glycol — —— — solvent Polyethylene glycol 600 7.5 15 — 7.5 Polyethylene glycolderivative — — — — Surfactant Acetylene glycol EO adduct(*) 0.1 0.1 0.10.1 Water soluble dye C. I. Direct Blue 199 0.5 0.5 0.5 0.5 Pure water34.4 34.4 34.4 34.4 Surface tension (mN/m) 39.5 39.6 39.4 40.1(*)Product name: Acetylenol EH manufactured by Kawaken Fine ChemicalsCo., Ltd.

<Measurement of Dot Diameter>

When the diameter of an ink dot obtained immediately after the inklanded on a plain paper is denoted by dI, the largest diameter of theink spread after the ink fixed on the paper is denoted by dS, thelargest diameter of the water-insoluble coloring material in the dotafter the ink fixed on the paper is denoted by dC, and the penetrationdepth of the water-insoluble coloring material in the paper after theink dried is represented by the penetration depth, the values of dI, dS,dC, and the penetration depth were measured by the following methods.First, the diameter dI of an ink dot immediately after the ink dotimpacts a plain paper was measured using Face CONTACT-ANGLEMETER CA-Pmanufactured by Kyowa Interface Science Co., Ltd. Using a needle havinga needle diameter of 28 G (inner diameter: 0.18 mm, and outer diameter:0.36 mm), the tip of the needle was located at a height of 4 mm from thesurface of the plain paper, ink was then dropped from the height ontothe plain paper, and after the dropping, the diameter of an ink dot wasread from the scale of the contact anglemeter. The above read value wasused as the ink dot diameter dI immediately after the ink landed on aplain paper. The plain paper used in this test was a PB-Paper (NSKpaper) manufactured by Canon Inc.

The largest diameter dS of the spread of the ink after the ink fixed ona recording medium, and the largest diameter dC of the water-insolublecoloring material in the ink after the ink fixed on a recording medium,were measured by dropping the ink dot on a plain paper under theabove-described conditions and leaving it for 6 hours or more, and bymeasuring Ace respective largest linear dimensions after the ink dot wasstabilized. The largest diameter dS of the ink dot after the ink fixedon a recording medium was obtained by measuring the largest lineardimension of the spread of cyan color of the water-soluble dye C. I.direct blue 199 that had been added to the ink. The largest diameter dCof the water-insoluble coloring material in the ink after the ink fixedon a recording medium was obtained by measuring the largest lineardimension of the spread of black hue of the pigment contained in theink. Furthermore, when the largest diameter dC of the spread of thewater-insoluble coloring material in the ink was measured, the dot formof the black pigment was also observed.

<Measurement of Penetration Depth of Water-Insoluble Coloring Material>

In order to measure the penetration depth of a water-insoluble coloringmaterial, the water-soluble dye C. I. direct blue 199 was replaced withwater in each of the inks of the above Example 6 and ComparativeExamples 18 to 20. The thus obtained inks were then filtrated under thesame conditions, so as to prepare new inks. Using these inks, printingwas carried out on a PB-Paper (NSK paper) manufactured by Canon Inc.,under the same conditions as in the above print density evaluation,using a modified ink jet recording apparatus BJS-700 (manufactured byCanon. Inc.) having an on-demand multi-recording head, which appliesthermals energy to ink in response to a recording signal so as to ejectthe ink. After completion of the printing and ink fixation, the printedarea wars sectioned from the backside thereof using a razor, and thecross section was observed with a microscope. The thickness (depth) ofthe water-insoluble coloring material distribution in the section of thepaper was measured. From the obtained measurement results, evaluationwas carried out in accordance with evaluation criteria described below.The obtained evaluation results and the measurement values of thelargest diameter dC are shown in Table 9.

[Evaluation Criteria]

(Dot diameter relationship)

◯: satisfying the relationship of dC<dI<dS

X: not satisfying the relationship of dC<dI<dS

(Dot form of water-insoluble coloring material)

1: Visual observation from the above of the paper

◯: the dot form of the water-insoluble coloring material is a perfectcircle and an edge thereof is sharp.

X: the dot form of the water-insoluble coloring material is not aperfect circle and an edge thereof is smeared.

2: Visual observation from the side of the paper

◯: the dot of the water-insoluble coloring material is present at almostthe level of the paper surface.

X: the dot of the water-insoluble coloring material is clearly risingfrom the surface of the paper.

(Penetration Depth of Water-Insoluble Coloring Material)

◯: less than 30 μm

X: 30 μm or larger TABLE 9 Measurements of ink landing and the largestdiameter of expansion of coloring material Com. Com. Com. Ex. 6 Ex. 18Ex. 19 Ex. 20 Dot diameter ◯ ◯ X ◯ relationship Dot shape 1. Observation◯ ◯ X ◯ of water- from the above insoluble 2. Observation ◯ X ◯ ◯coloring from the side material Penetration depth of ◯ ◯ X Xwater-insoluble coloring material The largest diameter dC 3.04 2.84 3.752.96 [mm] by spreading of water-insoluble coloring material

FIG. 15 schematically shows the look down view of the dots formed forthe above measurement. FIG. 16 is a schematically drawn photomicrographused to measure the expansion thickness (depth) of the water-insolublecoloring material in the section of the paper. As shown in FIG. 15 andTable 9, when the inks of Example 6 and Comparative Example 18 wereused, the water-insoluble coloring material formed a perfect circle dot.In the case of Comparative Example 18, however, the water-insolublecoloring the surface of the paper was covered with an excessive amountof water-insoluble coloring material. Hence, when the ink of Example 6was compared with that of Comparative Example 18, the coloring materialeffectively stayed near the surface of the paper. On the other hand,when the ink of Comparative Example 19 was used, the water-insolublecoloring material formed a dot with uneven spreading. Moreover, thewater-insoluble coloring material was spread not only on the surface ofthe paper, but also in the depth direction of the paper, causing loss inthe coloring material. Furthermore when the ink of Comparative Example20 was used, as shown in FIG. 15, the water-insoluble coloring materialformed a perfectly circular dot. However, as is clear from the sectionof a printed area shown in FIG. 16, which was printed using an ink jetrecording apparatus BJS-700 (manufactured by Canon Inc.), the coloringmaterial was spread not only on the surface of the paper, but also inthe depth direction of the paper. Thus, it was confirmed that in thiscase also, the coloring material was not effectively used.

As is apparent from the values of the largest diameter dC; of the spreadof the water-insoluble coloring material shown in Table 9, the ink ofExample 6 containing both a poor solvent and a good solvent at anappropriate ratio had a larger spread of the water-insoluble coloringmaterial than that of the ink of Comparative Example 18 containing onlya poor solvent. From this fact, it was also confirmed that the pigmentink of the present invention has a sufficiently large area factor evenwith a small of ink droplet and forms an image having high OD(reflection density) In addition, inks not containing a surfactant wereprepared by replacing the acetylene glycol EO adduct used as asurfactant with water in respective ink compositions of Example 6 andComparative Example 18. These inks were measured on the dot diameter andthe penetration depth of a water-insoluble coloring material under thesame conditions of landing measurement on a plain paper carried out withthe inks of Example 6 and Comparative Example 18. As a result, althoughthe inks containing no surfactant required a longer fixation time afterlanding on the paper than the inks containing a surfactant, the inkcontaining both a poor solvent and a good solvent at an appropriateratio achieved a larger spread of the water-insoluble coloring materialthan the ink containing only a poor solvent, providing the samerelationship as inks containing a surfactant.

[Test Regarding Color Mixing (Bleeding) of Image]

EXAMPLES 7 TO 16

The above-described inks of Examples 1 to 5 were used as the black inkin combination of color inks to form images. The color inks used herein(three colors, cyan, magenta and yellow) were prepared as follows.

(Preparation of Cyan Ink)

Components indicated below were mixed and fully stirred so that theywere dissolved, and thereafter, the mixture was subjected to pressurefiltration using a microfilter with a pore size of 0.2 gun (manufacturedby Fuji Photo Film Co., Ltd.), so as to prepare cyan ink. DBL (directblue) 199  3.5 parts Glycerin  7.5 parts Diethylene glycol  7.5 partsAcetylenol E-100  1.0 part Pure water 80.5 parts

(Preparation of Magenta Ink)

Magenta ink was prepared from the following components in the samemanner as for the cyan ink. AR (acid red) 289  2.5 parts Glycerin  7.5parts Diethylene glycol  7.5 parts Acetylenol E-100  1.0 part Pure water81.5 parts

(Preparation of Yellow Ink)

Yellow ink was prepared from the following components in the same maNNeras described above. DY (direct yellow) 86  2.5 parts Glycerin  7.5 partsDiethylene glycol  7.5 parts Acetylenol E-100  1.0 part Pure water 81.5parts

<Evaluation>

The black inks of Examples 1 to 5 and the above prepared colored inkswere used in the combination as shown in Table 10 below, and printingwas carried out by using these inks and an ink jet recording apparatushaving an on-demand multi-recording head shown in FIG. 9 or 10, whichapplies thermal energy to ink in response to a recording signal. so asto eject the ink. Thereafter, the print was evaluated. The obtainedevaluation results are shown in Table 11. TABLE 10 Types of the blackink and the head used for print evaluation Structure of head Black inkExample 7 Example 1 Example 8 Example 2 Example 9 Example 3 Example 10Example 4 Example 11 Example 5 Example 12 Example 1 Example 13 Example 2Example 14 Example 3 Example 15 Example 4 Example 16 Example 5

(Bleeding Properties)

The solid areas were formed by printing with a black ink and each ofcolor inks (yellow, magenta and cyan), such that the areas were adjacentto one another by the recording method shown in FIGS. 9 and 10. Thedegree of bleeding on the border between the black ink and the color inkwas visually observed, and evaluated in accordance with the criteriadescribed below. The plain paper used herein was a PB-Paper (NSK paper)manufactured by Canon Inc. The evaluation results are shown in Table 11.

AA: Bleeding cannot be visually observed.

A: Bleeding is hardly recognized.

B: Bleeding is recognized, but the level is actually not problematic.

C: Bleeding is such that the border of the colors is unclear. TABLE 11Evaluation results Bleed resistance Example 7 A Example 8 A Example 9 AExample 10 A Example 11 A Example 12 AA Example 13 AA Example 14 AAExample 15 AA Example 16 AA

Industrial Applicability

The water-based ink of the present invention is a pigmented ink whichhas a sufficiently large area factor even with a small ink droplet andachieves an image with high OD (reflection density) Moreover, using theink of the present invention, an ink jet recording method for forming ahigh quality image with high OD even with a small application amount ofthe ink, an ink cartridge preferably used for the above recordingmethod, a recording unit, and an ink jet recording apparatus are alsoprovided. Furthermore, there is also provided an image forming method inwhich color mixing (bleed) is effectively prevented on the borderbetween a black ink region and color ink region without causingfeathering, when a color image in which different colors are adjacent toone another is recorded on a plain paper

1. A water-based ink comprising water, a plurality of water-solubleorganic solvents, and a water-insoluble coloring material, wherein saidwater-soluble organic solvents are comprised of at least one goodsolvent to said water insoluble coloring material and at least one poorsolvent to said water-insoluble coloring material, and a total amount A(% by mass) of the good solvent or good solvents in the ink and a totalamount B (% by mass) of the poor solvent or poor solvents in the ink arein a ratio A:B of from 10:5 to 10:30 both inclusive, and thewater-soluble solvent having the largest Ka value determined by theBristow method among the water-soluble organic solvents is a poorsolvent.
 2. A water-based ink comprising water, a plurality ofwater-soluble organic solvents, and a water-insoluble coloring material,wherein said water-soluble organic solvents are comprised of at leastone good solvent to said water insoluble coloring material and at leastone poor solvent to said water-insoluble coloring material, thewater-soluble solvent having the largest Ka value determined by theBristow method among the water-soluble organic solvents is a poorsolvent, wherein when the ink is dropped on a plain paper from a needlewith a needle diameter of 28 G (inner diameter: 0.18 mm, and outerdiameter: 0.36 mm), the tip of the needle is located at a height of 4 mmfrom the surface of the plain paper, and the ink is then fixed on thesurface, a diameter (dI) of an ink dot obtained immediately after theink landed on the plain pmaper, a largest diameter (dS) of the ink dotafter the ink fixed on the plain paper, and a largest diameter (dC) ofan expansion of the water-insoluble coloring material in the ink dotafter the ink fixed on the plain paper satisfy a relation of Formula 1dC<dI<dS (Formula 1), and wherein when the ink is applied on a plainpaper by an ink jet recording, a penetration depth of thewater-insoluble coloring material into the plain paper is less than 30μm after the ink fixed.
 3. The water-based ink according to claim 2,wherein a total amount A (% by mass) of the good solvent or goodsolvents in the ink and the total amount B (% by mass) of the poorsolvent or poor solvents in the ink are in a ratio A:B of from 10:5 to10:30 both, inclusive.
 4. The water-based ink according to claim 1 or 2,wherein said water-insoluble coloring material is any one selected fromthe group consisting of a resin dispersed pigment using a resin as adispersant, a microencapsulated pigment, and a self-dispersing pigment.5. The water-based ink according to claim 1 or 2, wherein saidwater-insoluble coloring material is a self-dispersing organic pigmentor a self-dispersing carbon black and has at least one hydrophilic groupon the surface of particles thereof, wherein said hydrophilic group isbonded to the surface of the particles, directly or via another atomgroup.
 6. The water-based ink according to claim 5, wherein saidhydrophilic group is any one selected from the group consisting of-COOM1, —SO₃M1, and —PO₃H(M1)2 wherein M1 represents any one selectedfrom the group consisting of hydrogen, an alkali metal, ammonium, and anorganic ammonium.
 7. The water-based ink according to claim 5, whereinsaid other atom group is any one selected from the group consisting ofan alkylene group containing 1 to 12 carbon atoms, a substituted orunsubstituted phenylene group, and a substituted or unsubstitutednaphthylene group.
 8. The water-based ink according to claim 1 or 2,wherein said water-insoluble coloring material is a modified organicpigment or a carbon black in which an organic group is chemically bondedto the surface of the particles of the coloring material, wherein saidorganic group comprises a reaction product between a functional groupthat is bonded to the surface of the particles of the organic pigment orcarbon black, directly or chemically via another atom group, and acopolymer of an ionic monomer and a hydrophobic monomer.
 9. Thewater-based ink according to claim 1 or 2, wherein a Ka value determinedby the Bristow method is less than 1.5 (ml/m²/msec^(½)).
 10. Thewater-based ink according to claim 1 or 2, wherein a Ka value determinedby the Bristow method is 0.2 (ml/m²/msec^(½)) or more.
 11. Thewater-based ink according to claim 1 or 2, wherein said water-insolublecoloring material is between 0.1% and 15% by mass based on the totalamount of the ink.
 12. The water-based ink according to claim 11,wherein said water-insoluble coloring material is between 1% and 10% bymass based on the total amount of the ink.
 13. The water-based inkaccording to claim 1 or 2, which is used in ink jet printing.
 14. An inkjet recording method, which comprises at least a step of ejecting thewater-based ink according to claim 13 by the ink jet method.
 15. The inkjet recording method according to claim 14, wherein said ink jet methodis an ink jet recording method applying thermal energy to the ink togenerate an air bubble to eject an ink droplet.
 16. An ink cartridgecontaining a water-based ink according to any one of claim 1 or
 2. 17. Arecording unit comprising an ink-storing unit for storing a water-basedink according to claim 13 and an ink jet head for ejecting the ink. 18.An ink jet recording apparatus comprising an ink storing unit forstoring a water-based ink according to claim 13 and an ink jet head forejecting the ink.
 19. An image forming method by ink jet recording on aplain paper using a black ink and at least one water-based color ink toform an image consisting of a black image formed with a black ink thatis adjacent to a color image formed with a color ink, the methodcomprising the steps of: scanning for attaching a black ink to a plainpaper to form a black image; and scanning for attaching a color ink to aregion where the black image has already been formed wherein the blackink is a water-based ink according to claim
 13. 20. The image formingmethod according to claim 19, wherein the step of scanning for attachingthe color ink is carried out after at least one scan after the step ofscanning for attaching the black ink.
 21. The image forming methodaccording to claim 19, wherein ink is applied by using a recording headin which a line of discharge orifices for ejecting the black ink and aline of discharge orifices for ejecting the color inks are located witha shift in a direction of paper feeding.
 22. The image forming methodaccording to claim 19, wherein ink is applied by the ink jet recordingmethod involving giving thermal energy to the ink to generate an airbubble to eject an ink droplet.
 23. A water-based ink comprising water,a plurality of different water-soluble organic solvents, and awater-insoluble coloring material, wherein said water-soluble organicsolvents is are comprised of at least one good solvent to saidwater-insoluble coloring material and at least one poor solvent to saidwater-insoluble coloring material, and a water-soluble solvent havingthe largest Ka value determined by the Bristow method among thewater-soluble organic solvents is a poor solvent and the poor solventpermeates a recording medium before the good solvent does to assistagglomeration of the water-insoluble coloring material in the goodsolvent on the surface of the recording medium.
 24. The water-based inkaccording to claim 6, wherein said other atom group is any one selectedfrom the group consisting of an alkylene group containing 1 to 12 carbonatoms, a substituted or unsubstituted phenylene group, and a substitutedor unsubstituted naphthylene group.
 25. The water-based ink according toclaim 9, wherein a Ka value determined by the Bristow method is 0.2(ml/m²/msec^(½)) or more.